Implementing logic function and generating analog signals using NOR memory strings

ABSTRACT

NOR memory strings may be used for implementations of logic functions involving many Boolean variables, or to generate analog signals whose magnitudes are each representative of the bit values of many Boolean variables. The advantage of using NOR memory strings in these manners is that the logic function or analog signal generation may be accomplished within one simultaneous read operation on the NOR memory strings.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation-in-part application of U.S.non-provisional patent application Ser. No. 16/582,996 (“Non-provisionalApplication I”), entitled “Memory Circuit, System and Method for RapidRetrieval of Data Sets,” filed on Sep. 25, 2019, which is a continuationapplication of U.S. non-provisional patent application (“Non-ProvisionalApplication II”), Ser. No. 16/107,306, entitled “System Controller andMethod for Determining the Location of the Most Current Data File Storedon a Plurality of Memory Circuit,” filed on Aug. 21, 2018, which is adivisional application of U.S. non-provisional patent application(“Non-provisional Application III”), Ser. No. 15/248,420, entitled“Capacitive-Coupled Non-Volatile Thin-Film Transistor Strings in ThreeDimensional Arrays,” filed on Aug. 26, 2016, which is related to andclaims priority of (i) U.S. provisional application (“ProvisionalApplication I”), Ser. No. 62/235,322, entitled “Multi-gate NOR FlashThin-film Transistor Strings Arranged in Stacked Horizontal ActiveStrips With Vertical Control Gates,” filed on Sep. 30, 2015; (ii) U.S.provisional patent application (“Provisional Application II”), Ser. No.62/260,137, entitled “Three-dimensional Vertical NOR Flash Thin-filmTransistor Strings,” filed on Nov. 25, 2015; (iii) U.S. non-provisionalpatent application (“Non-Provisional Application IV”), Ser. No.15/220,375, “Multi-Gate NOR Flash Thin-film Transistor Strings Arrangedin Stacked Horizontal Active Strips With Vertical Control Gates,” filedon Jul. 26, 2016; and (vi) U.S. provisional patent application(“Provisional Application III”), Ser. No. 62/363,189, entitled“Capacitive Coupled Non-Volatile Thin-film Transistor Strings,” filedJul. 15, 2016.

The disclosures of Provisional Application I, Provisional ApplicationII, Non-Provisional Patent Application I-IV and Provisional ApplicationI-III are hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to high-density memory structures. Inparticular, the present invention relates to high-density, lowread-latency memory structures formed by interconnected thin-filmstorage elements (e.g., stacks of thin-film storage transistors, or“TFTs”, organized as NOR-type TFT strings or “NOR strings”).

2. Discussion of the Related Art

In this disclosure, memory circuit structures are described. Thesememory circuit structures may be fabricated on planar semiconductorsubstrates (e.g., silicon wafers) using conventional fabricationprocesses. To facilitate clarity in this description, the term“vertical” refers to the direction perpendicular to the surface of asemiconductor substrate, and the term “horizontal” refers to anydirection that is parallel to the surface of that semiconductorsubstrate.

A number of high-density non-volatile memory structures, sometimesreferred to as “three-dimensional vertical NAND strings,” are known inthe prior art. Many of these high-density memory structures are formedusing thin-film storage transistors (TFTs) formed out of depositedthin-films (e.g., polysilicon thin-films), and organized as arrays of“memory strings.” One type of memory strings is referred to as NANDmemory strings or simply “NAND strings”. A NAND string consists of anumber of series-connected TFTs. Reading or programming any of theseries-connected TFTs requires activation of all series-connected TFTsin the NAND string. Under this NAND arrangement, the activated TFTs thatare not read or programmed may experience undesirable program-disturb orread-disturb conditions. Further, TFTs formed out of polysilicon thinfilms have much lower channel mobility—and therefore higherresistivity—than conventional transistors formed in a single-crystalsilicon substrate. The higher series resistance in the NAND stringlimits the number of TFTs in a string in practice to typically no morethan 64 or 128 TFTs. The low read current that is required to beconducted through a long NAND string results in a long latency.

Another type of high-density memory structures is referred to as the NORmemory strings or “NOR strings.” A NOR string includes a number ofstorage transistors each of which is connected to a shared source regionand a shared drain region. Thus, the transistors in a NOR string areconnected in parallel, so that a read current in a NOR string isconducted over a much lesser resistance than the read current through aNAND string. To read or program a storage transistor in a NOR string,only that storage transistor needs to be activated (i.e., “on” orconducting), all other storage transistors in the NOR string may remaindormant (i.e., “off” or non-conducting). Consequently, a NOR stringallows much faster sensing of the activated storage transistor to beread. Conventional NOR transistors are programmed by a channelhot-electron injection technique, in which electrons are accelerated inthe channel region by a voltage difference between the source region andthe drain region and are injected into the charge-trapping layer betweenthe control gate and the channel region, when an appropriate voltage isapplied to the control gate. Channel hot-electron injection programmingrequires a relatively large electron current to flow through the channelregion, therefore limiting the number of transistors that can beprogrammed in parallel. Unlike transistors that are programmed byhot-electron injection, in transistors that are programmed byFowler-Nordheim tunneling or by direct tunneling, electrons are injectedfrom the channel region to the charge-trapping layer by a high electricfield that is applied between the control gate and the source and drainregions. Fowler-Nordheim tunneling and direct tunneling are orders ofmagnitude more efficient than channel hot-electron injection, allowingmassively parallel programming; however, such tunneling is moresusceptible to program-disturb conditions.

3-Dimensional NOR memory arrays are disclosed in U.S. Pat. No. 8,630,114to H. T Lue, entitled “Memory Architecture of 3D NOR Array”, filed onMar. 11, 2011 and issued on Jan. 14, 2014.

U.S. patent Application Publication US2016/0086970 A1 by Haibing Peng,entitled “Three-Dimensional Non-Volatile NOR-type Flash Memory,” filedon Sep. 21, 2015 and published on Mar. 24, 2016, discloses non-volatileNOR flash memory devices consisting of arrays of basic NOR memory groupsin which individual memory cells are stacked along a horizontaldirection parallel to the semiconductor substrate with source and drainelectrodes shared by all field effect transistors located at one or twoopposite sides of the conduction channel.

Three-dimensional NAND memory structures are disclosed, for example, inU.S. Pat. No. 8,878,278 to Alsmeier et al. (“Alsmeier”), entitled“Compact Three Dimensional Vertical NAND and Methods of Making Thereof,”filed on Jan. 30, 2013 and issued on Nov. 4, 2014. Alsmeier disclosesvarious types of high-density NAND memory structures, such as “terabitcell array transistor” (TCAT) NAND arrays (FIG. 1A), “pipe-shapedbit-cost scalable” (P-BiCS) flash memory (FIG. 1B) and a “vertical NAND”memory string structure. Likewise, U.S. Pat. No. 7,005,350 to Walker etal. (“Walker I”), entitled “Method for Fabricating Programmable MemoryArray Structures Incorporating Series—Connected Transistor Strings,”filed on Dec. 31, 2002 and issued on Feb. 28, 2006, also discloses anumber of three-dimensional high-density NAND memory structures.

U.S. Pat. No. 7,612,411 to Walker (“Walker II”), entitled “Dual-GateDevice and Method” filed on Aug. 3, 2005 and issued on Nov. 3, 2009,discloses a “dual gate” memory structure, in which a common activeregion serves independently controlled storage elements in two NANDstrings formed on opposite sides of the common active region.

U.S. Pat. No. 6,744,094 to Forbes (“Forbes”), entitled “Floating GateTransistor with Horizontal Gate Layers Stacked Next to Vertical Body”filed on May 3, 2004 and issued on Oct. 3, 2006, discloses memorystructures having vertical body transistors with adjacent parallelhorizontal gate layers.

U.S. Pat. No. 6,580,124 to Cleaves et al, entitled “MultigateSemiconductor Device with Vertical Channel Current and Method ofFabrication” filed on Aug. 14, 2000 and issued on Jun. 17, 2003,discloses a multi-bit memory transistor with two or four charge storagemediums formed along vertical surfaces of the transistor.

A three-dimensional memory structure, including horizontal NAND stringsthat are controlled by vertical polysilicon gates, is disclosed in thearticle “Multi-layered Vertical gate NAND Flash Overcoming StackingLimit for Terabit Density Storage” (“Kim”), by W. Kim at al., publishedin the 2009 Symposium on VLSI Tech. Dig. of Technical Papers, pp188-189. Another three-dimensional memory structure, also includinghorizontal NAND strings with vertical polysilicon gates, is disclosed inthe article, “A Highly Scalable 8-Layer 3D Vertical-gate (VG) TFT NANDFlash Using Junction-Free Buried Channel BE-SONOS Device,” by H. T. Lueet al., published in the 2010 Symposium on VLSI: Tech. Dig. Of TechnicalPapers, pp. 131-132.

U.S. Pat. No. 8,026,521 to Zvi Or-Bach et al, entitled “SemiconductorDevice and Structure,” filed on Oct. 11, 2010 and issued on Sep. 27,2011 to Zvi-Or Bach et al discloses a first layer and a second layer oflayer-transferred mono-crystallized silicon in which the first andsecond layers include horizontally oriented transistors. In thatstructure, the second layer of horizontally oriented transistorsoverlays the first layer of horizontally oriented transistors, eachgroup of horizontally oriented transistors having side gates.

In the memory structures discussed herein, stored information isrepresented by the stored electric charge, which may be introduced usingany of a variety of techniques. For example, U.S. Pat. No. 5,768,192 toEitan, entitled “Memory Cell Utilizing Asymmetrical Charge-trapping,”filed on Jul. 23, 1996 and issued on Jun. 16, 1998, discloses NROM typememory transistor operation based on the hot electron channel injectiontechnique.

Transistors that have a conventional non-volatile memory transistorstructure but short retention times may be referred to as“quasi-volatile.” In this context, conventional non-volatile memorieshave data retention time exceeding tens of years. A planarquasi-volatile memory transistor on single crystal silicon substrate isdisclosed in the article “High-Endurance Ultra-Thin Tunnel Oxide inMonos Device Structure for Dynamic Memory Application”, by H. C. Wannand C. Hu, published in IEEE Electron Device letters, Vol. 16, No. 11,November 1995, pp 491-493. A quasi-volatile 3-D NOR array withquasi-volatile memory is disclosed in the U.S. Pat. No. 8,630,114 to H.T Lue, mentioned above.

SUMMARY

According to one embodiment of the present invention, a NOR memorystring may be used to implement a logic function involving many Booleanvariables, or to generate an analog signal whose magnitude isrepresentative of the bit values of many Boolean variables. Theadvantage of using a NOR memory string in either of these manners isthat the logic function or the generation of the analog signal may beaccomplished in one read operation on the memory cells in the NOR memorystring.

According to one embodiment of the present invention, an array of memorycells includes TFTs formed in stacks of horizontal active strips runningparallel to the surface of a silicon substrate and control gates invertical local word lines running along one or both sidewalls of theactive strips, with the control gates being separated from the activestrips by one or more charge-storage elements. Each active stripincludes at least a channel layer formed between two shared source ordrain layers. The TFTs are organized as NOR strings. The TFTs associatedwith each active strip may belong to one or two NOR strings, dependingon whether one or both sides of each active strip are used.

In one embodiment, only one of the shared source or drain layers in anactive strip is connected by a conductor to a supply voltage through aselect circuit, while the other source or drain layer is held at avoltage determined by the quantity of charge that is provided to thatsource or drain layer. Prior to a read, write or erase operation, someor all of the TFTs in a NOR string along the active strip that are notselected for the read, write or erase operation act as a stripcapacitor, with the channel and source or drain layers of the activestrip providing one capacitor plate and the control gate electrodes inthe TFTs of the NOR string that are referenced to a ground referenceproviding the other capacitor plate. The strip capacitor is pre-chargedbefore the read, write or erase operation by turning on one or more TFTs(“pre-charge TFT”) momentarily to transfer charge to the strip capacitorfrom the source or drain layer that is connected by conductor to avoltage source. Following the pre-charge operation, the select circuitis deactivated, so that the pre-charged source or drain layer is heldfloating at substantially the pre-charged voltage. In that state, thecharged strip capacitor provides a virtual reference voltage source forthe read, write, or erase operation. This pre-charged state enablesmassively parallel read, write or erase operations on a large number ofaddressed TFTs. In this manner, TFT of many NOR strings on one or moreactive strips in one or more blocks of a memory array may be read,written or erased concurrently. In fact, blocks in a memory array can bepre-charged for program or erase operations, while other blocks in thememory array can be pre-charged for read operations concurrently.

In one embodiment, TFTs are formed using both vertical side edges ofeach active strip, with vertical local word lines being provided alongboth the vertical side edges of the active strips. In that embodiment,double-density is achieved by having the local word lines along one ofvertical edges of an active strip contacted by horizontal global wordlines provided above the active strip, while the local word lines alongthe other vertical edge of the active strip are contacted by horizontalglobal word lines provided beneath the active strip. All global wordlines may run in a direction transverse to the direction along thelengths of the corresponding active strips. Even greater storage densitymay be achieved by storing more than one bit of data in each TFT.

Organizing the TFTs into NOR strings in the memory array—rather than theprior art NAND strings—results in (i) a reduced read-latency thatapproaches that of a dynamic random access memory (DRAM) array, (ii)reduced sensitivities to read-disturb and program-disturb conditionsthat are known to be associated with long NAND strings, (iii) reducedpower dissipation and a lower cost-per-bit relative to planar NAND or3-D NAND arrays, and (iv) the ability to read, write or erase TFTs onmultiple active strips concurrently to increase data throughput.

According to one embodiment of the present invention, variations inthreshold voltages within NOR strings in a block may be compensated byproviding electrically programmable reference NOR strings within theblock. Effects on a read operation due to background leakage currentsinherent to NOR strings can be substantially eliminated by comparing thesensed result of the TFT being read and that of a concurrently read TFTin a reference NOR string. In other embodiments, the charge-storingelement of each TFT may have its structure modified to provide a highwrite/erase cycle endurance (albeit, a lower data retention time thatrequires periodic refreshing). In this detailed description, such TFTshaving a higher write/erase cycle endurance but a shorter retention timethan the conventional memory TFTs (e.g., TFTs in conventional NANDstrings) are referred to as being “quasi-volatile.” However, as thesequasi-volatile TFTs require refreshing significantly less frequentlythan a conventional DRAM circuit, the NOR strings of the presentinvention may be used in lieu of DRAM in some applications. Using theNOR strings of the present invention in DRAM applications allows asubstantially lower cost-per-bit figure of merit, as compared to theconventional DRAMs, and a substantially lower read-latency, as comparedto conventional NAND strings.

According to some embodiments of the present invention, the activestrips are manufactured in a semiconductor process in which the sourceor drain layers, and the channel layers are formed and annealedindividually for each plane in the stack. In other embodiments, thesource or drain layers are annealed either individually or collectively(i.e., in a single step for all the source or drain layers), prior toconcurrently forming the channel layers in a single step.

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a -1 is a conceptualized memory structure which illustrates anarray of memory cells being organized into planes (e.g., plane 110) andactive strips (e.g., active strip 112) in one memory array or block 100formed on substrate 101, according to embodiments of the presentinvention.

FIG. 1a -2 shows conceptualized memory structure in which the memorycells of memory array or block 100 of FIG. 1a -1 are alternativelyorganized into pages (e.g., page 113), slices (e.g., slice 114) andcolumns (e.g., column 115), according to one embodiment of the presentinvention.

FIG. 1b shows a basic circuit representation of four NOR string pairs,each NOR string pair being located in a respective one of four planes,according to one embodiment of the present invention; corresponding TFTsof each NOR string share common vertical local word lines.

FIG. 1c shows a basic circuit representation of four NOR strings, eachNOR string being located in a respective one of four planes, accordingto one embodiment of the present invention; corresponding TFTs of eachNOR string share common local word lines.

FIG. 2a shows a cross section in a Y-Z plane of semiconductor structure200, after active layers 202-0 to 202-7 (each separated from the nextactive layer respectively by isolation layers 203-0 to 203-7) have beenformed on semiconductor substrate 201, but prior to formation ofindividual active strips, in accordance with one embodiment of thepresent invention.

FIG. 2b -1 shows semiconductor structure 220 a having N⁺ sublayers 221and 223 and P⁻ sublayer 222; semiconductor structure 220 a may be usedto implement any of active layers 202-0 to 202-7 of FIG. 2a , inaccordance with one embodiment of the present invention.

FIG. 2b -2 shows semiconductor structure 220 b, which adds metallicsublayer 224 to semiconductor structure 220 a of FIG. 2b -1; metallicsublayer 224 is formed adjacent N⁺ sublayer 223, in accordance with oneembodiment of the present invention.

FIG. 2b -3 shows semiconductor structure 220 c, which adds metallicsublayers 224 to semiconductor structure 220 a of FIG. 2b -1, metallicsublayers 224 are each formed adjacent to either one of N⁺ sublayers 221or one of N⁺ sublayers 223, in accordance with one embodiment of thepresent invention.

FIG. 2b -4 shows semiconductor structure 220 a of FIG. 2b -1, afterpartial annealing by a shallow rapid laser anneal step (represented bylaser apparatus 207), in accordance with one embodiment of the presentinvention.

FIG. 2b -5 shows semiconductor structure 220 d of FIG. 2b -1, afterinclusion of additional ultra-thin sublayers 221-d and 223-d tosemiconductor structure 220 a of FIG. 2b -1, according to one embodimentof the present invention.

FIG. 2c shows cross section in a Y-Z plane of structure 200 of FIG. 2athrough buried contacts 205-0 and 205-1, which connect N⁺ sublayers 223of active layers 202-0 and 202-1 to circuitry 206-0 and 206-1 insemiconductor substrate 201.

FIG. 2d illustrates forming trenches 230 in structure 200 of FIG. 2a ,in a cross section in an X-Y plane through active layer 202-7 in oneportion of semiconductor structure 200 of FIG. 2 a.

FIG. 2e illustrates, in one portion of semiconductor structure 200 ofFIG. 2a , depositing charge-trapping layers 231L and 231R on oppositeside walls of the active strips along trenches 230 in a cross section inan X-Y plane through active layer 202-7.

FIG. 2f illustrates depositing conductor 208 (e.g., N⁺ or P⁺ dopedpolysilicon or metal) to fill trenches 230 of FIG. 2 e.

FIG. 2g shows, after photo-lithographical patterning and etching stepson the semiconductor structure of FIG. 2f , achieving local conductors(“word lines”) 208W and pre-charge word lines 208-CHG by removingexposed portions of the deposited conductor 208, and filling theresulting shafts 209 with an insulation material or alternatively,leaving the shafts as air gap isolation.

FIG. 2h shows a cross section in the Z-X plane through a row of localword lines 208W of FIG. 2g , showing active strips in active layers202-7 and 202-6.

FIG. 2i shows embodiment EMB-1 of the present invention, in which localword lines 208W of FIG. 2h are each connected to either one of globalword lines 208 g-a (routed in one or more conductive layers providedabove active layers 202-0 to 202-7), or one of global word lines 208 g-s(routed in one or more conductive layers provided below the activelayers and between active layer 202-0 and substrate 201) (see, also,FIG. 4a ).

FIG. 2i -1 shows a three-dimensional view of horizontal active layers202-4 to 202-7 of embodiment EMB-1 of FIG. 2i , with local word lines208W-s or local pre-charge word lines 208-CHG connected to global wordlines 208 g-s, and local word lines 208W-a connected to global wordlines 208 g-a, and showing each active layer as having its N⁺ layer 223(acting as a drain region) connected through select circuits to any ofvoltage supplies (e.g., V_(ss), V_(bl), V_(pgm), V_(inhibit), andV_(erase)), with decoding, sensing and other circuits arranged eitheradjacent or directly underneath the memory arrays; these circuits arerepresented schematically by circuitry 206-0 and 206-1 in substrate 201.

FIG. 2j shows embodiment EMB-2 of the present invention, in which onlytop global word lines 208 g-a are provided—i.e., without any bottomglobal word lines; in embodiment EMB-2, local word lines 208W-STG alongone edge of an active strip are staggered with respect to the local wordlines 208W-a along the opposite edge of the active strip (see, also,FIG. 4b ).

FIG. 2k shows embodiment EMB-3 of the present invention, in which eachof local word lines 208W controls a pair of TFTs (e.g., TFTs 281 and283) formed in opposing side walls of adjacent active strips and theirrespective adjacent charge-trapping layers (e.g., trapping layers 231Land 231R); isolation trenches 209 are etched to isolate each TFT pair(e.g., TFTs 281 and 283) from adjacent TFT pairs (e.g., TFTs 285 and287) (see, also, FIG. 4c ).

FIG. 2k -1 shows embodiment EMB-3 of FIG. 2k , in which optional P-dopedpillars 290 are provided to fill part or all of isolation trenches 209,so as to selectively connect P⁻ sublayers 222 to substrate circuits;P-doped pillars 290 may supply back-bias voltage V_(bb) or erase voltageV_(erase) to P⁻ sublayers 222 (see, also, FIGS. 3a -1 and 4 c).

FIG. 3a -1 illustrates the methods and circuit elements used for settingsource voltage V_(ss) in N⁺ sublayers 221; specifically, source voltageV_(ss) may be set through hard-wire decoded source line connections 280(shown in dashed line) or alternatively, by activating pre-charge TFTs303 and decoded bit line connections 270 to any one of voltage sourcesfor bit line voltages V_(ss), V_(pgm), V_(inhibit) and V_(erase).

FIG. 3a -2 shows the circuit of FIG. 3a -1, for the case when metallicsublayer 224 is provided along the length of N⁺ sublayer 223 to providea low-resistance signal path.

FIG. 3b shows exemplary waveforms of the source, drain, selected wordline and non-selected word line voltages for the circuit of FIG. 3a -1during a read operation, in which N⁺ sublayer 221 is applied sourcevoltage V_(ss) through hard-wired connections 280.

FIG. 3c shows exemplary waveforms for the source, drain, selected wordline, non-selected word line and pre-charge word line voltages for thecircuit of FIG. 3a -1 during a read operation, in which N⁺ sublayer 221provides a semi-floating source region after being momentarilypre-charged to V_(ss) (˜0V) by pre-charge word line 208-CHG, with thenon-selected word line 151 b being held at ˜0V.

FIG. 4a is a cross section in the X-Y plane of embodiment EMB-1 of FIGS.2i and 2i -1, showing contacts 291 connecting local word lines 208W-a toglobal word lines 208 g-a at the top of the memory array; likewise,local word lines 208W-s are connected to global word lines 208 g-s (notshown) running at the bottom of the memory array substantially parallelto the top global word line.

FIG. 4b is a cross section in the X-Y plane of embodiment EMB-2 of FIG.2j , showing contacts 291 connecting local word lines 208W-a andstaggered local word lines 208W-STG to either top global word lines 208g-a only, or alternatively, to bottom global word lines only (not shown)in a staggered configuration of TFTs along both sides of each activestrip.

FIG. 4c is a cross section in the X-Y plane of embodiment (EMB-3) ofFIGS. 2k and 2k -1, showing contacts 291 connecting local word lines208W-a to global word lines 208 g-a at the top of the memory array, oralternatively, to global word lines 208 g-s at the bottom of the array(not shown), with isolation trenches 209 separating TFT pair 281 and 283from TFT pair 285 and 287 on adjacent active strips in active layer202-7.

FIG. 4d is a cross section in the X-Y plane of embodiment EMB-3 of FIGS.2k and 2k -1 through active layer 202-7, additionally including one ormore optional P-doped pillars 290 which provide to P⁻ sublayers 222,selectively, substrate back-bias voltage V_(bb) and erase voltageV_(erase).

FIG. 5a shows a cross section through a Y-Z plane of semiconductorstructure 500, after horizontal active layers 502-0 through 502-7 havebeen formed, one on top of each other, and isolated from each other byrespective isolation layers 503-0 to 503-7 (of material ISL) onsemiconductor substrate 201.

FIG. 5b is a cross section in a Y-Z plane through buried contacts 205-0and 205-1, through which N⁺ sublayers 523-1 and 523-0 are respectivelyconnected to circuitry 206-0 and 206-1 in semiconductor substrate 201.

FIG. 5c is a cross section in the Z-X plane, showing planes or activelayers 502-6 and 502-7 of structure 500 after trenches 530 along theY-direction are anisotropically etched through active layers 502-7 to502-0 to reach down to landing pads 264 of FIG. 5b ; the SAC2 materialfilling trenches 530 has etch characteristics that are different fromthose of the SAC1 material.

FIG. 5d shows the top plane or active layer 502-7 in an X-Y planethrough sublayer 522 of the SAC1 material, showing secondary trench 545etched anisotropically into the SAC2 material that fills trenches 530,reaching the bottom of the stack of active layers 502-7 to 502-0; theanisotropic etch exposes sidewalls 547 of the stacks to allow etchant toetch away the SAC1 material to make room for sublayer 522 by forming acavity between N⁺ sublayer 521 and N⁺ sublayer 523 in each active stripof active layers 502-0 to 502-7.

FIG. 5e is a cross section through the Z-X plane (e.g., along line 1-1′of FIG. 5d ) away from trench 545, showing active strips in adjacentactive layers supported by the SAC2 material on both sides of eachactive strip; in cavities 537, resulting from excavating the SAC1material in sublayer 522, optional ultra-thin dopant diffusion-blockinglayer 521-d is provided, over which is deposited undoped or P⁻ dopedpolysilicon 521.

FIG. 5f illustrates, in a cross section in the X-Y plane of embodimentEMB-1A of the present invention, P-doped pillars 290, local word lines280W and pre-charge word lines 208-CHG being provided between and alongadjacent active strips of active layer 502-7, the word lines beingformed after the SAC2 material in trenches 530 are selectively removed;prior to forming the word lines, charge-trapping layers 231L and 231Rare deposited conformally on the side walls of the active strips(Ultra-thin dopant diffusion-blocking layer 521-d is optional).

FIG. 5g shows a cross section in the Z-X plane of active layers 502-6and 502-7 of embodiment EMB-3A, after formation of optional ultra-thindopant diffusion blocking layer 521-d and deposition of undoped or P⁻doped polysilicon, amorphous silicon, or silicon germanium in sublayer522 that forms the channel regions of TFTs T_(R) 585, T_(R) 587; thesublayer 522 (P⁻) is also deposited on the trench side walls as pillars290 to connect the channel regions in the stack (i.e., P⁻ sublayer 522)to substrate circuitry 262.

FIG. 5h -1 shows cross section 500 in the Z-X plane, showing activestrips immediately prior to etching the sacrificial SAC1 materialbetween N⁺ sublayers 521 and 522, in accordance with one embodiment ofthe present invention.

FIG. 5h -2 shows cross section 500 of FIG. 5h -1, after sidewayselective etching of the SAC1 material (along the direction indicated byreference numeral 537) to form selective support spines out of the SAC1material (e.g., spine SAC1-a), followed by filling the recesses with P⁻doped material (e.g., P⁻ doped polysilicon) and over the sidewalls ofthe active strips, according to one embodiment of the present invention.

FIG. 5h -3 shows cross section 500 of FIG. 5h -2, after removal of theP⁻ material from areas 525 along the sidewalls of the active strips,while leaving P⁻ sublayer 522 in the recesses, in accordance with oneembodiment of the present invention; FIG. 5h -3 also shows removal ofisolation materials from trenches 530, formation of charge-trappinglayer 531 and local word lines 208-W, thus forming transistors T_(L) 585and T_(R) 585 on opposite sides of the active strips.

FIG. 6a shows semiconductor structure 600, which is a three-dimensionalrepresentation of a memory array organized into quadrants Q1-Q4; in eachquadrant, (i) numerous NOR strings are each formed in an active stripextended along the Y-direction (e.g., NOR string 112), (ii) pagesextending along the X-direction (e.g., page 113), each page consistingof one TFT from each NOR string at a corresponding Y-position, the NORstrings in the page being of the same corresponding Z-position (i.e., ofthe same active layer); (iii) slices extending in both the X- andZ-directions (e.g., slice 114), with each slice consisting of the pagesof the same corresponding Y-position, one page from each of the planes,and (iv) planes extending along both the X- and Y-directions (e.g.,plane 110), each plane consisting of all pages at a given Z-position(i.e., of the same active layer).

FIG. 6b shows structure 600 of FIG. 6a , showing TFTs in programmablereference string 112-Ref in quadrant Q4 and TFTs in NOR string 112 inquadrant Q2 coupled to sense amplifiers SA(a), Q2 and Q4 being “mirrorimage quadrants”; FIG. 6b also shows (i) programmable reference slice114-Ref (indicated by area A) in quadrant Q3 similarly providingcorresponding reference TFTs for slice 114 in mirror image quadrant Q1,sharing sense amplifiers SA(b), and (ii) programmable reference plane110-Ref in quadrant Q2 providing corresponding reference TFTs to plane110 in mirror image quadrant Q1, sharing sense amplifiers SA(c), andalso providing corresponding reference TFTs for NOR strings in the samequadrant (e.g., NOR string 112).

FIG. 6c shows structure 600 of FIG. 6a , showing slices 116 being usedas a high speed cache because of their close proximity to their senseamplifiers and voltage sources 206; FIG. 6c also show spare planes 117,which may be used to provide replacement or substitution NOR strings orpages in quadrant Q2.

FIG. 7 is a cross section in the Z-X plane of active layer 502-7 ofembodiment EMB-3A, showing in greater detail short-channel TFT T_(R) 585of FIG. 5g , in which N⁺ sublayer 521 serves as source and N⁺ sublayer523 serves as drain and P⁻ sublayer 522 serves as channel in conjunctionwith charge storage material 531 and word line 208W; FIG. 7 demonstratesan erase operation in which electrons trapped in storage material 531(e.g., in regions 577 and 578) are removed to N⁺ sublayer 521 and N⁺sublayer 523, assisted by fringing electric field 574.

FIG. 8a shows in simplified form prior art storage system 800 in whichmicroprocessor (CPU) 801 communicates with system controller 803 in aflash solid state drive (SSD) that employs NAND flash chips 804; the SSDemulates a hard disk drive and NAND flash chips 804 do not communicatedirectly with CPU 801 and have relatively long read latency.

FIG. 8b shows in simplified form system architecture 850 using thememory devices of the present invention, in which non-volatile NORstring arrays 854, or quasi-volatile NOR string arrays 855 (or both)communicate directly with CPU 801 through one or more input and output(I/O) ports 861, and indirectly through controller 863.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1a -1 and 1 a-2 show conceptualized memory structure 100,illustrating in this detailed description an organization of memorycells according to embodiments of the present invention. As shown inFIG. 1a -1, memory structure 100 represents a 3-dimensional memory arrayor block of memory cells formed in deposited thin-films fabricated overa surface of substrate layer 101. Substrate layer 101 may be, forexample, a conventional silicon wafer used for fabricating integratedcircuits, familiar to those of ordinary skill in the art. In thisdetailed description, a Cartesian coordinate system (such as indicatedin FIG. 1a -1) is adopted solely for the purpose of facilitatingdescription. Under this coordinate system, the surface of substratelayer 101 is considered a plane which is parallel to the X-Y plane.Thus, as used in this description, the term “horizontal” refers to anydirection parallel to the X-Y plane, while the term “vertical” refers tothe Z-direction. As shown, block 100 consists of four planes (e.g.,plane 110) stacked in the vertical direction one on top of, and isolatedfrom, each other. Each plane consists of horizontal active strips of NORstrings (e.g., active strip 112). Each NOR string includes multiple TFTs(e.g., TFT 111) formed side-by-side along the active strip, withthin-film transistor current flowing in the vertical direction, asdescribed in further detail below. Unlike prior art NAND strings, in theNOR string of the present invention, writing, reading or erasing one ofthe TFTs in the NOR string does not require activating other TFTs in theNOR string. Accordingly, each NOR string is randomly addressable and,within such a NOR string, each TFT is randomly accessible.

Plane 110 is shown as one of four planes that are stacked on top of eachother and isolated from each other. Along the length of horizontalactive strip 112 are formed side-by-side TFTs (e.g., TFT 111). In FIG.1a -1, for illustrative purpose only, each plane has four horizontalactive strips that are isolated from each other. Both the plane and theNOR strings are individually addressable.

FIG. 1a -2 introduces additional randomly addressable units of memorycells: “columns,” “pages” and “slices”. In FIG. 1a -2, each column(e.g., column 115) represents TFTs of multiple NOR strings that share acommon control gate or local word line, the NOR strings are formed alongactive strips of multiple planes. Note that, as a conceptualizedstructure, memory structure 100 is merely an abstraction of certainsalient characteristics of a memory structure of the present invention.Although shown in FIG. 1a -1 as an array of 4×4 active strips, eachhaving four TFTs along their respective lengths, a memory structure ofthe present invention may have any number of TFTs along any of the X-,Y- and Z-directions. For example, there may be 1, 2, 4, 8, 16, 32, 64 .. . planes of strings in the Z direction, 2, 4, 8, 16, 32, 64, . . .active strips of NOR strings along the X-direction, and each NOR stringmay have 2, 4, 8, 16, . . . 8192 or more side-by-side TFTs in theY-direction. The use of numbers that are integer powers of 2 (i.e.,2^(n), where n is an integer) follows a customary practice inconventional memory design. It is customary to access each addressableunit of memory by decoding a binary address. Thus, for example, a memorystructure of the present invention may have M NOR strings along each ofthe X and Z directions, with M being a number that is not necessarily2^(n), for any integer n. TFTs of structure 100 of the present inventioncan be read, programmed or erased simultaneously on individual page orindividual slice basis. (As shown in FIG. 1a -2, a “page” refers to arow of TFTs along the Y-direction; a “slice” refers to an organizationof contiguous memory cells that extend along both the X- andZ-directions and one memory cell deep along the Y-direction). An eraseoperation can also be performed in one step for entire memory block 100.

As a conceptualized structure, memory structure 100 is not drawn toscale in any of the X-, Y-, and Z-directions.

FIG. 1b shows a basic circuit representation of four NOR string pairs,each NOR string pair being located in a respective one of four planes,according to one embodiment of the present invention; corresponding TFTsof each NOR string share common local word lines (e.g., local word line151 n). The detailed structure of this configuration is discussed andillustrated below in conjunction with FIG. 2k . As shown in FIG. 1b ,this basic circuit configuration includes four NOR string pairs on fourseparate planes (e.g., NOR strings 150L and 150R in plane 159-4) thatare provided in adjacent columns 115 of memory structure 100 sharing acommon local word line.

As shown in FIG. 1b , NOR strings 150L and 150R may be NOR stringsformed along two active strips located on opposite sides of shared localword line 151 a. TFTs 152R-1 to 152R-4 and 152L-1 to 152L-4 may be TFTslocated in the four active strips and the four active strips on oppositesides of local word line 151 a, respectively. In this embodiment, asillustrated in greater detail below in conjunction with FIG. 2k and FIG.4c , a greater storage density may be achieved by having a sharedvertical local word line control TFTs of adjacent active strips. Forexample, local word line 151 a controls TFTs 152R-1, 152R-2, 152R-3 and152R-4 from four NOR strings located on four planes, as well as TFTs152L-1, 152L-2, 152L-3 and 152L-4 from four adjacent NOR strings oncorresponding planes. As discussed in greater detail below, in someembodiments, the parasitic capacitance C intrinsic to each NOR string(e.g., the distributed capacitance between the common N⁺ source regionor N⁺ drain region of a NOR string and its multiple associated localword lines) may be used as a virtual voltage source, under someoperating conditions, to provide source voltage V_(ss).

FIG. 1c shows a basic circuit representation of four NOR strings, eachNOR string being located in a respective one of four planes, accordingto one embodiment of the present invention. In FIG. 1c , correspondingTFTs of each NOR string share common local word lines. Each NOR stringmay run horizontally along the Y-direction, with storage elements (i.e.,TFTs) connected between source line 153-m and drain or bit lines 154-m,where m is the index between 1 to 4 of the corresponding active strip,with drain-source transistor currents flowing along the Z-direction.Corresponding TFTs in the 4 NOR strings share corresponding one of localword lines 151-n, where n is the index of a local word line. The TFTs inthe NOR strings of the present invention are variable threshold voltagethin-film storage transistors that may be programmed, program-inhibited,erased, or read using conventional programming, inhibition, erasure andread voltages. In one or more embodiments of the present invention, theTFTs are implemented by thin-film storage transistors that areprogrammed or erased using Fowler-Nordheim tunneling or direct tunnelingmechanisms. In another embodiment, channel hot-electron injection may beused for programming

Process Flow

FIG. 2a shows a cross section in a Y-Z plane of semiconductor structure200, after active layers 202-0 to 202-7 (each separated from the nextactive layer respectively by isolation layers 203-0 to 203-7) have beenformed on semiconductor substrate 201, but prior to formation ofindividual active strips, in accordance with one embodiment of thepresent invention. Semiconductor substrate 201 represents, for example,a P⁻ doped bulk silicon wafer on which support circuits for memorystructure 200 may be formed prior to forming the active layers. Suchsupport circuits, which may be formed alongside contacts 206-0 and 206-1in FIGS. 2c and 2i -1, may include both analog and digital circuits.Some examples of such support circuits include shift registers, latches,sense amplifiers, reference cells, power supply lines, bias andreference voltage generators, inverters, NAND, NOR, Exclusive-Or andother logic gates, input/output drivers, address decoders (e.g., bitline and word line decoders), other memory elements, sequencers andstate machines. These support circuits may be formed out of the buildingblocks for conventional devices (e.g., N-wells, P-wells, triple wells,N⁺, P⁺ diffusions, isolation regions, low and high voltage transistors,capacitors, resistors, vias, interconnects and conductors), as is knownto those of ordinary skill in the art.

After the support circuits have been formed in and on semiconductorsubstrate 201, isolation layer 203-0 is provided, which may be adeposited or grown thick silicon oxide, for example.

Next, in some embodiments, one or more layers of interconnect may beformed, including “global word lines,” which are further discussedbelow. Such metallic interconnect lines (e.g., global word line landingpads 264 of FIG. 2c , discussed below) may be provided as horizontallong narrow conductive strips running along a predetermined directionthat may be perpendicular to the active NOR strings to be formed at alater step. To facilitate discussion in this detailed description, theglobal word lines are presumed to run along the X-direction. Themetallic interconnect lines may be formed by applyingphoto-lithographical patterning and etching steps on one or moredeposited metal layers. (Alternatively these metallic interconnect linescan be formed using a conventional damascene process, such as a copperor Tungsten damascene process). A thick oxide is deposited to formisolation layer 203-0, followed by a planarization step usingconventional chemical mechanical polishing (CMP) techniques.

Active layers 202-0 to 202-7 are then successively formed, each activelayer being electrically insulated from the previous active layerunderneath by a corresponding one of isolation layers 203-1 to 203-7. InFIG. 2a , although eight active layers are shown, any number of activelayers may be provided. In practice, the number of active layers maydepend on the process technology, such as availability of awell-controlled anisotropic etching process that allows cutting througha tall stack of the active layers to reach semiconductor substrate 201.Each active layer is etched at an etching step that preferentially cutsthrough the planes as discussed below to form a large number of parallelactive strips each running along the Y-direction.

FIG. 2b -1 shows semiconductor structure 220 a having N⁺ sublayers 221and 223 and P⁻ sublayer 222. Semiconductor structure 220 a may be usedto implement any of active layers 202-0 to 202-7 of FIG. 2a , inaccordance with one embodiment of the present invention. As shown inFIG. 2b -1, active layer 220 a includes deposited sublayers 221-223 ofpolysilicon. In one implementation, sublayers 221-223 may be depositedsuccessively in the same process chamber without removal in between.Sublayer 223 may be formed by depositing 10-100 nm of in-situ doped N⁺polysilicon. Sublayers 222 and 221 may then be formed by depositingundoped or lightly doped polysilicon or amorphous silicon, in thethickness range of 10-100 nm. Sublayer 221 (i.e., the top portion of thedeposited polysilicon) is then N⁺ doped. N⁺ dopant concentrations insublayers 221 and 223 should be as high as possible, for example between1×10²⁰/cm³ and 1×10²¹/cm³, to provide the lowest possible sheetresistivity in N⁺ sublayers 221 and 223. The N⁺ doping may be achievedby either (i) a low-energy shallow high-dose ion implantation ofphosphorus, arsenic or antimony, or (ii) in-situ phosphorus or arsenicdoping of the deposited polysilicon, forming a 10-100 nm thick N⁺sublayer 221 on top. Low-dose implantations of boron (P⁻) or phosphorus(N⁻) ions may also be carried out at energies sufficient to penetratethe implanted or in-situ doped N⁺ sublayer 221 into sublayer 222 lyingbetween N⁺ sublayer 221 and N⁺ sublayer 223, so as to achieve anintrinsic enhancement mode threshold voltage in the resulting TFTs. Theboron or P⁻ dopant concentration of sublayer 222 can be in the range of1×10¹⁶/cm³ to 1×10¹⁸/cm³; the actual boron concentration in sublayer 222determines the native transistor turn-on threshold voltage, channelmobility, N⁺P⁻N⁺ punch-through voltage, N⁺P⁻ junction leakage andreverse diode conduction characteristics, and channel depletion depthunder the various operating conditions for the N⁺P⁻N⁺ TFTs formed alongactive strips 202-0 to 202-7.

Thermal activation of the N⁺ and P⁻ implanted species andrecrystallization of sublayers 221, 222 and 223 should preferably takeplace all at once after all active layers 202-0 to 202-7 have beenformed, using a conventional rapid thermal annealing technique (e.g., at700° C. or higher) or a conventional rapid laser annealing technique,thereby ensuring that all active layers experience elevated temperatureprocessing in roughly the same amount. Caution must be exercised tolimit the total thermal budget, so as to avoid excessive diffusion ofthe dopants out of N⁺ sublayer 223 and sublayer 221, resulting ineliminating form the TFTs P sublayer 222, which acts as a channelregion. P⁻ sublayer 222 is required to remain sufficiently thick, orsufficiently P-doped to avoid N⁺P⁻N⁺ transistor punch-through orexcessive leakage between N⁺ sublayer 221 and N⁺ sublayer 223.

Alternatively, N⁺ and P⁻ dopants of each of active layers 202-0 to 202-7can be activated individually by shallow rapid thermal annealing using,for example, excimer laser anneal (ELA) at an ultraviolet wavelength(e.g., 308 nanometer). The annealing energy which is absorbed by thepolysilicon or amorphous silicon to partially melt sublayer 221 and partor all of sublayer 222, optionally penetrating into sublayer 223 toaffect volume 205 (see FIG. 2b -4) without unduly heating other activelayers lying below sublayer 223 of the annealed active layer 220 a.

Although the use of successive layer-by-layer excimer laser shallowrapid thermal anneal is more costly than a single deep rapid thermalanneal step, ELA has the advantage that the localized partial melting ofpolysilicon (or amorphous silicon) can result in recrystallization ofannealed volume 205 to form larger silicon polycrystalline grains havingsubstantially improved mobility and uniformity, and reduced TFT leakagedue to reduced segregation of N⁺ dopants at the grain boundaries of theaffected volume. The ELA step can be applied either to P⁻ sublayer 222and N⁺ sublayer 223 before formation of N⁺ sublayer 221 above it, orafter formation of a sufficiently thin N⁺ sublayer 221 to allowrecrystallization of both sublayers 221 and 222 and, optionally,sublayer 223. Such shallow excimer laser low-temperature annealtechnique is well-known to those of ordinary skill in the art. Forexample, such technique is used to form polysilicon or amorphous siliconfilms in solar cell and flat panel display applications. See, forexample, H. Kuriyama et al. “Comprehensive Study of Lateral Grain Growthin Poly-Si Films by Excimer Laser Annealing (ELA) and its applicationsto Thin Film Transistors”, Japanese Journal of Applied Physics, Vol. 33,Part 1, Number 10, 20 Aug. 1994, or “Annealing of Silicon Backplaneswith 540 W Excimer Lasers”, technical publication by Coherent Inc. ontheir website.

The thickness of P⁻ sublayer 222 roughly corresponds to the channellength of the TFTs to be formed, which may be as little as 10 nm or lessover long active strips. In one embodiment (see FIG. 2b -5), it ispossible to control the channel length of the TFT to less than 10 nm,even after several thermal process cycles, by depositing an ultra-thin(from one or a few atomic layers to 3 nm thick) film of silicon nitride(e.g., SiN or Si₃N₄), or another suitable diffusion-blocking filmfollowing the formation of N⁺ sublayer 223 (see sublayer 223-d in FIG.2b -5). A second ultra-thin film of silicon nitride, or another suitablediffusion-blocking film (see 221-d in FIG. 2b -5), may optionally bedeposited following deposition of P⁻ sublayer 222, before depositing N⁺sublayer 221. The ultra-thin dopant diffusion-blocking layers 221-d and223-d can be deposited by chemical vapor deposition, atomic layerdeposition or any other suitable means (e.g., high pressurenitridization at low temperature). Each ultra-thin dopantdiffusion-blocking layer acts as a barrier that prevents the N⁺ dopantsin N⁺ sublayers 221 and 223 from diffusing into P⁻ sublayer 222, yet aresufficiently thin to only marginally impede the MOS transistor action inthe channel region between N⁺ sublayer 221 (acting as a source) and N⁺sublayer 223 (acting as a drain). (Electrons in the surface inversionlayer of sublayer 222 readily tunnel directly through the ultra-thinsilicon nitride layers, which are too thin to trap such electrons).These additional ultra-thin dopant diffusion-blocking layers increasethe manufacturing cost, but may serve to significantly reduce thecumulative leakage current from the multiple TFTs along the activestrips that are in the “off” state. However, if that leakage current istolerable then these ultra-thin layers can be omitted.

NOR strings having long and narrow N⁺ sublayers 223 and N⁺ sublayers 221may have excessively large line resistance (R), including the resistanceof narrow and deep contacts to the substrate. Reduced line resistance isdesirable, as it reduces the “RC delay” of a signal traversing a longconductive strip. (RC delay is a measure of the time delay that is givenby the product of the line resistance R and the line capacitance C).Reduced line resistance also reduces the “IR voltage drop” across a longand narrow active strip. (The IR voltage drop is given by the product ofthe current I and the line resistance R). To significantly reduce theline resistance, an optional conductive sublayer 224 may be added toeach active strip adjacent one or both of N⁺ sublayers 221 or 223 (e.g.,sublayer 224, labeled as W in FIGS. 2b -2 and 2 b-3). Sublayer 224 maybe provided by one or more deposited metal layers. For example, sublayer224 may be provided by depositing 1-2 nm thick layer of TiN followed bydepositing a 1-40 nm thick layer of tungsten, a similar refractorymetal, or a polycide or silicide (e.g., nickel silicide). Sublayer 224is more preferably in the 1-20 nm thickness range. Even a very thinsublayer 224 (e.g., 2-5 nm) can significantly reduce the line resistanceof a long active strip, while allowing the use of less heavily doped N⁺sublayers 21 and 223.

As shown in FIG. 2c , the conductor inside contact opening 205-1 canbecome quite long for a tall stack, thereby adversely increasing theline resistance. In that case, metallic sublayer layer 224 (e.g., atungsten layer) may preferably be included below sublayer 223, so as tosubstantially fill contact opening 205-1, rather than placing it aboveN⁺ sublayer 221, as is shown in FIG. 2c . Including metal sublayer 224in each of active layers 202-0 to 202-7 may, however, increase cost andcomplexity of the manufacturing process, including the complication thatsome of the metallic materials are relatively more difficult to etchanisotropically than materials such as polysilicon, silicon oxide orsilicon nitride. However, metallic sublayer 224 enables use ofconsiderably longer active strips, which results in superior arrayefficiency.

In the embodiments where no metallic sublayers 224 are incorporated,there are several tradeoffs that can be made: for example, longer activestrips are possible if the resultant increased read latency isacceptable. In general, the shorter the active strip, the lower the lineresistance and therefore the shorter the latency. (The trade-off is inarray efficiency). In the absence of metallic sublayer 224, thethickness of N⁺ sublayers 221 and 223 can be increased (for example to100 nanometers) to reduce the intrinsic line resistance, at the expenseof a taller stack to etch through. The line resistance can be furtherreduced by increasing the N⁺ doping concentration in N⁺ sublayers 221and 223 and by applying higher anneal temperatures in excess of 1,000°C. (e.g., by rapid thermal anneal, deep laser anneal or shallow excimerlaser anneal) to enhance recrystallization and dopant activation and toreduce dopant segregation at the grain-boundaries.

Shorter active strips also have superior immunity to leakage between N⁺sublayer 223 and N⁺ sublayer 221. A thicker N⁺ sublayer provides reducedstrip line resistance and increased strip capacitance, which isdesirable for dynamic sensing (to be discussed below). The integratedcircuit designer may opt for a shorter active strip (with or withoutmetal sublayer 224) when low read latency is most valued. Alternatively,the strip line resistance may be reduced by contacting both ends of eachactive strip, rather than just at one end.

Block-formation patterning and etching steps define separate blocks ineach of the active layers formed. Each block occupies an area in which alarge number (e.g., thousands) of active strips running in parallel maybe formed, as discussed below, with each active strip running along theY-direction, eventually forming one or more NOR strings that eachprovide a large number (e.g., thousands) of TFTs.

Each of active layers 202-0 to 202-7 may be successively formed byrepeating the steps described above. In addition, in the block-formationpatterning and etching steps discussed above, each next higher activelayer may be formed with an extension slightly beyond the previousactive layer (see, e.g., as illustrated in FIG. 2c , discussed below,layer 202-1 extends beyond layer 202-0) to allow the upper active layerto access its specific decoders and other circuitry in semiconductorsubstrate 201 through designated buried contacts.

As shown in FIG. 2c , buried contacts 205-0 and 205-1 connect contacts206-0 and 206-1 in semiconductor substrate 201, for example, to thelocal bit lines or source lines formed out of N⁺ sublayer 223 in each ofactive layers 202-0 and 202-1. Buried contacts for active layers 202-2to 202-7 (not shown) may be similarly provided to connect active layers202-2 to 202-7 to contacts 206-2 to 206-7 in semiconductor substrate 201in an inverted staircase-like structure in which the active layerclosest to the substrate has the shortest buried contact, while theactive layer furthest from the substrate has the longest buried contact.Alternatively, in lieu of buried contacts, conductor-filled viasextending from the top of the active layers may be etched throughisolation layers 203-0 and 203-1. These vias establish electricalcontact from substrate circuitry 206-0, for example, to top N⁺ sublayers221-0 (or metal sublayer 224, if provided). The vias may be laid out ina “staircase” pattern with the active layer closest the substrateconnected by the longest via, and the active layer closest to the topconnected by the shortest via. The vias (not shown) have the advantagethat more than one plane can be contacted in one masking-and-etch step,as is well-known to a person of ordinary skill in the art.

Through a switch circuit, each of contacts 206-0 to 206-7 may apply apre-charge voltage V_(bl) to the respective bit line or source line ofthe corresponding NOR strings or, during a read operation, may beconnected to an input terminal of a sense amplifier or a latch. Theswitch circuit may selectively connect each of contacts 206-0 to 206-7to any of a number of specific voltage sources, such as a programmingvoltage (V_(pgm)), inhibit voltage (V_(inhibit)), erase voltage(V_(erase)), or any other suitable predetermined or pre-charge referencevoltage V_(bl) or V_(ss). In some embodiments, discussed below, takingadvantage of the relatively large parasitic distributed capacitancealong a bit line or source line in an active strip, a virtual voltagereference (e.g., a virtual ground, providing ground voltage V_(ss)) maybe created in the source line (i.e., N⁺ sublayer 221) of each activestrip by pre-charging the source line, as discussed below. The virtualground eliminates the need for hard-wiring N⁺ sublayer 221 to a voltagesource in the substrate, making it possible to use the staircase viastructure described above to connect each active strip from the top tothe substrate. Otherwise, it would be impossible to separately connectN⁺ sublayer 221 and N⁺ sublayer 223 of each active strip from the top tothe substrate, as the via material will short the two sublayers.

FIG. 2c also shows buried contacts 261-0 to 261-n for connecting globalword lines 208 g-s—which are to be formed running along theX-direction—to contacts 262-0 to 262-n in semiconductor substrate 201.Global word lines 208 g-s are provided to connect corresponding localword lines 208W-s yet to be formed (see, e.g., FIG. 2i ) to circuits262-n in substrate 201. Landing pads 264 are provided on the global wordlines to allow connection to local word lines 208W-s, which are yet tobe formed vertically on top of horizontally running global word lines208 g-s. Through a switch circuit and a global word line decoder, eachof global word line contacts 262-0 to 262-n may be selectivelyconnected, either individually, or shared among several global wordlines, to any one of a number of reference voltage sources, such asstepped programming voltages (V_(program)), program-inhibit voltage(V_(inhibit)), read voltages (V_(read)) and erasure voltages(V_(erase)).

The buried contacts, the global word lines and the landing pads may beformed using conventional photo-lithographical patterning and etchingsteps, followed by deposition of one or more suitable conductors or byalloying (e.g., tungsten metal, alloy or tungsten silicide).

After the top active layer (e.g., active layer 202-7) is formed,trenches are created by etching through the active layers to reach thebottom global word lines (or semiconductor substrate 201) using astrip-formation mask. The strip-formation mask consists of a pattern ina photoresist layer of long narrow strips running along the Y-direction.Sequential anisotropic etches etch through active layers 202-7 to 202-0,and dielectric isolations layers 203-7 to 203-0. As the number of activelayers to be etched, which is eight in the example of FIG. 2c (and, moregenerally may be 16, 32, 64 or more), a photoresist mask may not besufficiently robust to hold the strip-formation pattern through thenumerous etches necessary to etch through to beyond the lowest activelayer. Thus, reinforced masks using a hard mask material (e.g., carbonor a metal) may be required, as is known to those of ordinary skill inthe art. Etching terminates at the dielectric isolation layer above thelanding pads of the global word lines. It may be advantageous to providean etch-stop barrier film (e.g., an aluminum oxide film) to protect thelanding pads during the trench etch sequence.

FIG. 2d illustrates forming trenches 230 in structure 200 of FIG. 2a ,in a cross section in an X-Y plane through active layer 202-7 in oneportion of semiconductor structure 200 of FIG. 2a . Between adjacenttrenches 230 are high aspect-ratio, long and narrow active strips in thedifferent active layers. To achieve the best etch result, etch chemistrymay have to be changed when etching through the materials of thedifferent sublayers, especially in embodiments where metal sublayers 224are present. The anisotropy of the multi-step etch is important, asundercutting of any sublayer should be avoided, and so that an activestrip in the bottom active layer (e.g., an active strip in active layer202-0) has approximately the same width and gap spacing to an adjacentactive strip as the corresponding width and gap spacing in an activestrip in the top active layer (i.e., an active strip of active layer202-7). Naturally, the greater the number of active layers in the stackto be etched, the more challenging is the design of the successiveetches. To alleviate the difficulty associated with etching through alarge number of active layers (e.g., 32), etching may be conducted ingroups of layers, say 8, as discussed in Kim, referenced above, at pp.188-189.

Thereafter, one or more charge-trapping layers are conformally depositedor grown on the sidewalls of the active strips in trenches 230. Thecharge-trapping layer is formed by first chemically depositing orgrowing a thin tunneling dielectric film of a 2-10 nm thickness (e.g., asilicon dioxide layer, a silicon oxide-silicon nitride-silicon oxide(“ONO”) triple layer, a bandgap engineered nitride layer or a siliconnitride layer), preferably 3 nm or less, followed by deposition of a4-10 nm thick layer of charge-trapping material (e.g., silicon nitride,silicon-rich nitride or oxide, nanocrystals, nanodots embedded in a thindielectric film, or isolated floating gates), which is then capped by ablocking dielectric film. The blocking dielectric film may be a 5-15 nmthick layer consisting of, for example, an ONO layer, or a highdielectric constant film (e.g., aluminum oxide, hafnium oxide or somecombination thereof). The storage element to be provided can be SONOS,TANOS, nanodot storage, isolated floating gates or any suitablecharge-trapping sandwich structures known to those of ordinary skill inthe art.

Trenches 230 are formed sufficiently wide to accommodate the storageelements on the two opposing sidewalls of the adjoining active strips,plus the vertical local word lines to be shared between the TFT's onthese opposite sidewalls. FIG. 2e illustrates, in one portion ofsemiconductor structure 200 of FIG. 2a , depositing charge-trappinglayers 231L and 231R on opposite side walls of the active strips alongtrenches 230 in a cross section in an X-Y plane through active layer202-7.

Contact openings to the bottom global word lines are thenphoto-lithographically patterned at the top of layer 202-7 and exposedby anisotropically etching through the charge-trapping materials at thebottom of trenches 230, stopping at the bottom global word line landingpads (e.g., global word line landing pads 264 of FIG. 2c ). In oneembodiment, to be described in conjunction with FIG. 2i below, onlyalternate rows of trenches 230 (e.g., the rows in which the word linesformed therein are assigned odd-numbered addresses) are etched down tothe bottom global word lines. In some embodiments, etching is precededby a deposition of an ultra-thin sacrificial film (e.g. a 2-5 nm thickpolysilicon film) to protect the vertical surface of the blockingdielectric on the sidewalls of trenches 230 during the anisotropic etchof the charge-trapping material at the bottom of trenches 230. Theremaining sacrificial film can be removed by a short-duration isotropicetch.

Thereafter, doped polysilicon (e.g., P⁺ polysilicon or N⁺ polysilicon)may be deposited over the charge-trapping layers to form the controlgates or vertical local word lines. P⁺ doped polysilicon may bepreferable because of its higher work function compared to N⁺ dopedpolysilicon. Alternatively, a metal with a high work function relativeto SiO₂ (e.g., tungsten, tantalum, chrome, cobalt or nickel) may be usedto form the vertical local word lines. Trenches 230 may now be filledwith the P⁺ doped polysilicon or the metal. In the embodiment of FIG. 2i, discussed below, the doped polysilicon or metal in alternate rows oftrenches 230 (i.e., the rows to host local word lines 208W-s that areassigned odd-numbered addresses) is in ohmic contact with the bottomglobal word lines 208 g-s. The polysilicon in the other ones of trenches230 (i.e., the rows to host local word lines 208W-a that are assignedeven-numbered addresses) are isolated from the bottom global word lines.(These local word lines are to be later contacted by top global wordlines 208 g-a routed above the top active layer). The photoresist andhard mask may now be removed. A CMP step may then be used to remove thedoped polysilicon from the top surface of each block. FIG. 2fillustrates depositing conductor 208 (e.g., polysilicon or metal) tofill trenches 230 of FIG. 2 e.

FIG. 2g shows, after photo-lithographical patterning and etching stepson the semiconductor structure of FIG. 2f , achieving local conductors(“word lines”) 208W and pre-charge word lines 208-CHG by removingexposed portions of the deposited conductor 208, and filling theresulting shafts 209 with an insulation material or alternatively,leaving the shafts as air gap isolation. As removing doped polysiliconin this instance is a high aspect-ratio etch step in a confined space, ahard mask material (e.g., carbon or metal) may be required, using thetechnique described above. The resulting shafts 209 may be filled withinsulating material or may be left as air gaps to reduce parasiticcapacitance between adjacent local word lines. The mask pattern thatexposes the doped polysilicon for excavation are parallel strips thatrun along the X-direction, so that they coincide with the global wordlines 208 g-a that are required to be formed to contact local word lines208W-a (see FIG. 2i ) and local pre-charge word lines 208-CHG.

In FIG. 2g , portions 231X of charge-trapping layers 231L and 231Radjacent insulation shafts 209 remain after the removal of thecorresponding portions of deposited polysilicon 208W. In someembodiments, portions 231X of charge-trapping layers 231L and 231R maybe removed by a conventional etching process step prior to fillingshafts 209 with insulation material or air gap. Etching of thecharge-trapping materials in the shafts may be carried out concurrentlywith the removal of the doped polysilicon, or subsequent to it. Asubsequent etch would also remove any fine polysilicon stringers leftbehind by the anisotropic etch; these polysilicon stringers may causeundesirable leakage paths, serving as resistive leakage paths betweenadjacent local word lines. Removing part or all such charge-trappingmaterials at portions 231X eliminates parasitic edge TFTs as well asimpeding potential lateral diffusion of trapped charge between adjacentTFTs along the same NOR string. Partial removal of portions 231X can beaccomplished by a short-duration isotropic etch (e.g., a wet etch or aplasma etch), which removes the blocking dielectric film and part or allof the charge-trapping material not protected by the local word lines.

FIG. 2h shows a cross section in the Z-X plane through a row of localword lines 208W of FIG. 2g , showing active strips in active layers202-7 and 202-6. As shown in FIG. 2h , each active layer includes N⁺sublayer 221, P⁻ sublayer 222, and N⁺ sublayer 223 (low-resistivitymetal layer 224 is optional). In one embodiment, N⁺ sublayer 221 (e.g.,a source line) is hard-wire connected to ground reference voltage V_(ss)(shown in FIG. 3a -1 as ground reference voltage 280) and N⁺ sublayer223 (e.g., a bit line) is connected to a contact in substrate 201according to the method illustrated in FIG. 2c . Thus, local word line208W, the portion of active layer 202-7 or 202-6 facing word line 208Wand the charge-trapping layer 231L between word line 208W and thatportion of active layer 202-7 or 202-6 form the storage elements (e.g.,storage TFTs 281 and 282) in FIG. 2h . Facing TFTs 281 and 282 on theopposite side of local word line 208W are TFTs 283 and 284 respectively,incorporating therein charge-trapping layer 231R. On the other side ofthe active strips 202-6 and 202-7 providing TFTs 283 and 284 are TFTs285 and 286. Accordingly, the configuration shown in FIG. 2h representsthe highest packing density configuration for TFTs, with each local wordline shared by the two active strips along its opposite sides, and witheach active strip being shared by the two local word lines along its twoopposite side edges. Each local word line 208W may be used to read,write or erase the charge stored in the designated one of the TFTsformed in each of active layers 202-0 to 202-7, located on eithercharge-trapping portion 231L or 231R, when a suitable voltage isimposed.

N⁺ sublayer 223 (i.e., a bit line) can be charged to a suitable voltagerequired for an operation of the TFTs at hand (e.g., program voltageV_(prog), inhibition voltage V_(inhibit), erase voltage V_(erase), orthe read reference voltage V_(bl)). During a read operation, any of TFTs281-286 that are in the “on” state conduct current in the vertical orZ-direction between sublayers 221 and 223.

As shown in the embodiment of FIG. 2h , optional metal sublayer 224reduces the resistance of N⁺ sublayer 223, so as to facilitate fastmemory device operations. In other modes of operations, N⁺ sublayer 221in any of active layers 202-0 to 202-7 may be left floating. In eachactive layer, one or more of the local word lines (referred to as a“pre-charge word line”; e.g., pre-charge word lines 208-CHG in FIG. 2g )may be used as a non-memory TFT. When a suitable voltage is applied tothe pre-charge word lines (i.e., rendering the pre-charge TFTconducting), each pre-charge word line momentarily inverts its channelsublayer 222, so that N⁺ sublayer 221 (the source line) may bepre-charged to the pre-charge voltage V_(ss) in N⁺ sublayer 223, whichis supplied from voltage source V_(bl) in the substrate. When thevoltage on the pre-charge word line is withdrawn, (i.e., when thepre-charge TFT is returned to its non-conducing state) and all the otherword lines on both sides of the active strip are also “off”, deviceoperation may proceed with N⁺ sublayer 221 left electrically charged toprovide a virtual voltage reference at the pre-charged voltage V_(ss)(typically ˜0V) because the distributed parasitic capacitor formedbetween the N⁺ sublayer 221 and its multiple local word lines issufficiently large to hold its charge long enough to support theprogram, program-inhibit or read operation (see below). Although theTFTs in a NOR string may also serve as pre-charge TFTs along each NORstring, to speed up the pre-charge for read operations (read pre-chargerequires lower word line voltages of typically less than ˜5 volts), someof the memory TFTs (e.g., one in every 32 or 64 memory TFTs along theNOR string) may also be activated. It is preferable that, at least forhigh voltage pre-charge operations, TFTs that are dedicated entirely toserve as pre-charge TFTs are provided, as the they are more tolerant ofprogram-disturb conditions than the memory TFTs.

Alternatively, in one embodiment to be described below (e.g., embodimentEMB-3 shown in FIGS. 2k and 2k -1), each local word line 208W may beused to read, write or erase the TFTs formed in each of active layers202-0 to 202-7, located on either charge-trapping portions 231L or 231R,when a suitable voltage is imposed. However, as shown in FIG. 2k , onlyone of the two sides of each active strip in active layers 202-0 to202-7 is formed as storage TFTs, thereby eliminating the need for bothbottom and top global word lines in this specific embodiment.

An isolation dielectric or oxide may then be deposited and its surfaceplanarized. Contacts to semiconductor substrate 201 and to local wordlines 208W may then be photo-lithographically patterned and etched.Other desirable back-end processing beyond this step is well known to aperson of ordinary skill in the art.

Some Specific Embodiments of the Present Invention

In embodiment EMB-1, shown in FIGS. 2i and 4a , each of local word lines208W is connected to either one of global word lines 208 g-a (routed inone or more layers provided above active layers 202-0 to 202-7), or oneof global word lines 208 g-s (routed in one or more layers providedbelow the active layers between active layer 202-0 and substrate 201).Local word lines 208W-s that are coupled to bottom global word lines 208g-s may be assigned odd addresses, while local word lines 208W-a coupledto the top global word lines 208 g-a may be assigned even addresses, orvice versa. FIG. 4a is a cross section in the X-Y plane of embodimentEMB-1 of FIGS. 2i and 2i -1, showing contacts 291 connecting local wordlines 208W-a to global word lines 208 g-a at the top of the memoryarray. Likewise, local word lines 208W-s are connected to global wordlines 208 g-s (not shown) running at the bottom of the memory arraysubstantially parallel to the top global word line.

FIG. 2i -1 shows a three-dimensional view of horizontal active layers202-4 to 202-7 of embodiment EMB-1 of FIG. 2i , with local word lines208W-s or local pre-charge word lines 208-CHG connected to global wordlines 208 g-s and local word lines 208W-a connected to global word lines208 g-a, and showing each active layer as having its N⁺ layer 223(acting as a drain region) connected through select circuits to any ofvoltage supplies (e.g., V_(ss), V_(bl), V_(pgm), V_(inhibit), andV_(erase)), decoding, sensing and other circuits arranged eitheradjacent or directly underneath the memory arrays. The substratecircuitry is represented schematically by 206-0 and 206-1 in substrate201.

Each active strip is shown in FIG. 2i -1 with its N⁺ sublayer 223connected to substrate contacts 206-0 and 206-1 (V_(bl)), and P-sublayer222 (channel region) connected to substrate back-bias voltage (V_(bb))source 290 through circuitry 262-0. N⁺ sublayer 221 and optional lowresistivity metallic sublayer 224 may be hard-wired (see, e.g., groundreference connections 280 in FIG. 3a -1) to a V_(ss) voltage supply, oralternatively, it may be left floating, after being pre-chargedmomentarily to virtual source voltage V_(ss) through local pre-chargeword line 208-CHG. Global word lines 208 g-a at the top of the memoryarray and global word lines 208 g-s at the bottom of the memory arraymay make contact with vertical local word lines 208W-a and 208W-s andpre-charge word lines 208-CHG. Charge-trapping layers 231L and 231R areformed between the vertical local word lines and the horizontal activestrips, thus forming non-volatile memory TFTs at the intersection ofeach horizontal active strip and each vertical word line, on both sidesof each active strip. Not shown are isolation layers between activestrips on different planes and between adjacent active strips within thesame plane.

N⁺ sublayer 221 is either hard-wire connected to a ground voltage (notshown), or is not directly connected to an outside terminal and leftfloating, or pre-charged to a voltage (e.g., a ground voltage) during aread operation. Pre-charging may be achieved by activating localpre-charge word lines 208-CHG. P⁻ sublayer 222 of each active layer(providing the channel regions of TFTs) is optionally selectivelyconnected through pillars 290 (described below) to supply voltage V_(bb)in substrate 201. Metallic sublayer 224 is an optional low resistivityconductor, provided to reduce the resistivity of active layers 202-4 to202-7. To simplify, interlayer isolation layers 203-0 and 203-1 of FIG.2c are not shown.

Global word lines 208 g-a on top of the memory array are formed bydepositing, patterning and etching a metal layer following the formationof contacts or vias. Such a metal layer may be provided by, first,forming a thin tungsten nitride (TiN) layer, followed by forming a lowresistance metal layer (e.g., metallic tungsten). The metal layer isthen photo-lithographically patterned and etched to form the top globalword lines. (Alternatively, these global word lines may be provided by acopper damascene process.) In one implementation, these global wordlines are horizontal, running along the X-direction and electricallyconnecting the contacts formed in the isolation oxide (i.e., therebycontacting local word lines 208W-a or 208W-CHG) and with the contacts tosemiconductor substrate 201 (not shown). Other mask and etch processflows known to those of ordinary skill in the art are possible to formeven and odd addressed local word lines and connect them appropriatelyto their global word lines, either from the top of the memory arraythrough the top global word lines or from the bottom of the memory arraythrough the bottom global word lines (and, in some embodiments, fromboth top and bottom global word lines).

FIG. 2j shows embodiment EMB-2 of the present invention, in which onlytop global word lines 208 g-a are provided—i.e., without any bottomglobal word lines. In embodiment EMB-2, pre-charge local word lines208W-STG along one edge of an active strip are staggered with respect tothe local word lines 208W-a along the opposite edge of the active strip(see, also, FIG. 4b ). FIG. 4b is a cross section in the X-Y plane ofembodiment EMB-2 of FIG. 2j , showing contacts 291 connecting local wordlines 208W-a and staggered local word lines 208W-STG to either topglobal word lines 208 g-a only, or alternatively, to bottom global wordlines only (not shown) in a staggered configuration of TFTs along bothsides of each active strip.

Staggering the local word lines simplifies the process flow byeliminating the process steps needed to form the bottom global wordlines (or the top global word lines, as the case may be). The penaltyfor the staggered embodiment is the forfeiting of the double-densityTFTs inherent in having both edges of each active strip provide TFTswithin one pitch of each global word line. Specifically, in embodimentEMB-1 of FIG. 2i and corresponding FIG. 4a , in which both top andbottom global word lines are provided, two TFTs may be included in eachactive strip of each active layer within one pitch of a global word line(i.e., in each active strip, one TFT is formed using one sidewall of theactive strip, and controlled from a bottom global word line, the otherTFT is formed using the other sidewall of the active strip, andcontrolled from a top global word line). (A pitch is one minimum linewidth plus a required minimum spacing between adjacent lines). Bycontrast, as shown in FIG. 2j and corresponding FIG. 4b , in embodimentEMB-2, only one TFT may be provided within one global word line pitch ineach active layer. The local word lines 208W at the two sides of eachactive strip are staggered relative to each other to allow space for thetwo global word line pitches required to contact them both.

FIG. 2k shows embodiment EMB-3 of the present invention, in which eachof local word lines 208W controls a pair of TFTs (e.g., TFTs 281 and283) formed in opposing side walls of adjacent active strips and theirrespective adjacent charge-trapping layers (e.g., trapping layers 231Land 231R). Isolation trenches 209 are etched to isolate each TFT pair(e.g., TFTs 281 and 283) from adjacent TFT pairs (e.g., TFTs 285 and287) (see, also, FIG. 4c ). As shown in FIG. 2k , each TFT is formedfrom one or the other of a dual-pair of active strips located onopposite side of a shared local word line, with each dual-pair of activestrips separated from similarly formed adjacent dual-pairs of activestrips by isolation trenches 209 which, unlike trenches 230 do notprovide for TFTs on the opposite edges of each active strip (see, FIG.4c ). Trenches 209 may be filled with a dielectric isolation material(e.g., silicon dioxide, or charge-trapping material 231), or be left asan air gap. There is no accommodation therein for a local word line.

FIG. 4c is a cross section in the X-Y plane of embodiment (EMB-3) ofFIGS. 2k and 2k -1, showing contacts 291 connecting local word lines208W-a to global word lines 208 g-a at the top of the memory array, oralternatively, to global word lines 208 g-s at the bottom of the array(not shown), with isolation trenches 209 separating TFT pair 281 and 283from TFT pair 285 and 287 on adjacent active strips in active layer202-7.

Alternatively, isolation trenches 209 can include pillars of P⁻ dopedpolysilicon (e.g., pillars 290 in FIG. 2k -1 and FIG. 4d ) connected tothe substrate to provide back-bias supply voltage V_(bb) (also shown asvertical connections 290 in FIG. 3a -1). Pillars 290 supply back-biasvoltages (e.g., V_(bb) 0V to 2V) during read operations to reducesub-threshold source-drain leakage currents. Alternatively, pillar 290may supply back-bias voltage V_(bb) and an erase voltage V_(erase) (˜12Vto 20V) during erase operations. Pillars 290 can be formed as isolatedvertical columns as shown in FIG. 4d , or they can fill part or all ofthe length of each of trenches 209 (not shown). Pillars 290 contact P⁻sublayers 222 in all active layers 202-0 to 202-7. However, pillars 290cannot be provided in embodiments where metallic sublayers 224 areprovided because such an arrangement may result in paths of excessiveleakage currents between different planes.

FIG. 4d is a cross section in the X-Y plane of embodiment EMB-3 of FIGS.2k and 2k -1 through active layer 202-7, additionally including one ormore optional P-doped pillars 290 which provide selectively substrateback-bias voltage V_(bb) and erase voltage V_(erase) to P⁻ sublayers222.

FIG. 3a -1 illustrates the methods and circuit elements used for settingsource voltage V_(ss) in N⁺ sublayers 221. Specifically, source voltageV_(ss) may be set through hard-wire decoded source line connections 280(shown in dashed line) or alternatively, by activating pre-charge TFTs303 and decoded bit line connections 270 to any one of bit line voltagesV_(ss), V_(bl), V_(pgm), V_(inhibit) and V_(erase). FIG. 3a -2 shows thecircuit of FIG. 3a -1, for the case when metallic sublayer 224 isprovided along the length of N⁺ sublayer 223 to provide a low-resistancesignal path. The sheet resistivity of N⁺ sublayer 221 establishes theincremental electrical resistance R along its length that issubstantially proportional to the distance from ground connector node280. Alternatively, source reference voltage V_(ss) may be accessedthrough a metal or N⁺ doped polysilicon conductor connecting from thetop of the memory array through staircase vias, in the manner commonlyemployed in prior art 3D NAND stacks. Each of the conductors inhard-wired connections 280 may be independently connected, so that thesource voltages for different planes or within planes need not be thesame. The requirement for hard-wired conductors to connect N⁺ sublayer221 to the reference voltage V_(ss) necessitates additional patterningand etching steps for each of active layers 202-0 to 202-7, as well asadditional address decoding circuitry, thereby increasing complexity andmanufacturing cost. Hence in some embodiments, it is advantageous todispense with the hard-wired source voltage V_(ss) connections, bytaking advantage of a virtual voltage source in the intrinsic parasiticcapacitance of the NOR string, as discussed below.

Dynamic Operation of Nor Strings

The present invention takes advantage of the cumulative intrinsicparasitic capacitance that is distributed along each NOR string todramatically increase the number of TFTs that can be programmed, read orerased in parallel in a single operation, while also significantlyreducing the operating power dissipation, as compared to 3-D NAND flasharrays. As shown in FIG. 3a -1, local parasitic capacitor 360(contributing to a cumulative capacitance C) exists at each overlapbetween a local word line (as one plate) and the N⁺/P⁻/N⁺ active layer(as the other plate). For the TFTs of the NOR strings with minimumfeature size of 20 nanometers, each local parasitic capacitor isapproximately 0.005 femtofarads (each femtofarad is 1×10⁻¹⁵ farad), toosmall to be of much use for temporary storage of charge. However, sincethere may be a thousand or more TFTs contributing capacitance from oneor both sides of an active strip, the total distributed capacitance C ofN⁺ sublayer 221 (the source line) and N⁺ sublayer 223 (the bit line) ina long NOR string can be in the range of ˜1 to 20 femtofarads. This isalso roughly the capacitance at sensing circuitry connected throughconnections 270 (e.g., voltage source V_(bl)).

Having the bit line capacitance of the NOR string almost the same valueas the parasitic capacitance of the source line (where charge istemporarily stored) provides a favorable signal-to-noise ratio during asensing operation. In comparison, a DRAM cell of the same minimumfeature size has a storage capacitor of approximately 20 femtofarads,while its bit line capacitance is around 2,000 femtofarads, or 100 timesthat of its storage capacitor. Such mismatch in capacitance results in apoor signal-to-noise ratio and the need for frequent refreshes. A DRAMcapacitor can hold its charge for typically 64 milliseconds, due toleakage of the capacitor's charge through the DRAM cell's accesstransistor. In contrast, the distributed source line capacitance C of aNOR string has to contend with charge leakage not just of one transistor(as in a DRAM cell), but the much larger charge leakage through thethousand or more parallel unselected TFTs. This leakage occurs in TFTson word line 151 b (WL-nsel) of FIG. 3a -1 that share the same activestrip as the one selected TFT on word line 151 a (WL-sel) and reducessubstantially the charge retention time on the distributed capacitance Cof the NOR string to perhaps a few hundred microseconds, thus requiringmeasures to reduce or neutralize the leakage, as discussed below.

As discussed below, the leakage current due to the thousand or moretransistors occurs during read operations. During program,program-inhibit or erase operations, both N⁺ sublayers 221 and 223 arepreferably held at the same voltage, therefore the leakage currentbetween the two N⁺ sublayers 221 and 223 is insignificant. Duringprogram, program-inhibit or erase operations, charge leakage fromcumulative capacitance C flows primarily to the substrate through thesubstrate selection circuitry, which has very little transistor leakage,as it is formed in single crystal or epitaxial silicon. Nevertheless,even a 100-microsecond charge retention time is sufficient to completethe sub-100 nanosecond read operation or the sub-100 microsecond programoperation (see below) of the selected TFT on the NOR string.

A TFT in a NOR string, unlike a DRAM cell, is a non-volatile memorytransistor, so that, even if parasitic capacitor C of the NOR string iscompletely discharged, the information stored in the selected TFTremains intact in the charge storage material (i.e., charge-trappinglayer 231). This is the case for all the NOR strings of embodimentsEMB-1, EMB-2, and EMB-3. In a DRAM cell, however, the information wouldbe forever lost without frequent refreshes. Accordingly, distributedcapacitance C of a NOR string of the present invention is used solely totemporarily hold the pre-charge voltage on N⁺ sublayers 221 and 223 atone of voltages V_(ss), V_(bl), V_(progr), V_(inhibit), or V_(erase),and not used to store actual data for any of the TFTs in the NOR string.Pre-charge transistor 303 of FIG. 3a -1, controlled by word line 151 n(i.e., word line 208-CHG), is activated momentarily immediatelypreceding each read, program, program-inhibit or erase operation totransfer voltage V_(bl) (e.g., through connections 270) from thesubstrate circuitry (not shown) to N⁺ sublayer 221. For example, voltageVim can be set at ˜0V to pre-charge N⁺ sublayer 221 to a virtual groundvoltage ˜0V during a read operation, or to pre-charge both N⁺ sublayers221 and 223 to between ˜5V and ˜10V during a program inhibit operation.

The value of cumulative capacitance C may be increased by lengtheningthe NOR string to accommodate the thousands more TFTs along each side ofthe active strip, correspondingly increasing the retention time ofpre-charge voltage V_(ss) on N⁺ sublayer 221. However, a longer NORstring suffers from an increased line resistance as well as higherleakage currents between N⁺ sublayer 221 and N⁺ sublayer 223. Suchleakage currents may interfere with the sensed current when reading theone TFT being addressed with all other TFT's of the NOR string in their“off” (and somewhat leaky) states. Also, the potentially longer time ittakes to pre-charge a larger capacitor during a read operation canconflict with the desirability for a low read latency (i.e., a fast readaccess time). To speed up the pre-charging of the cumulative capacitanceC of a long NOR string, pre-charge TFTs may be provided spaced apartalong either side of the active strip (e.g., once every 128, 256 or moreTFTs).

Because the variable-threshold TFTs in a long NOR string are connectedin parallel, the read operating condition for the NOR string shouldpreferably ensure that all TFTs along both edges of an active stripoperate in enhancement mode (i.e., they each have a positive thresholdvoltage, as applied between control gate 151 n and voltage V_(ss), atsource 221). With all TFTs being in enhancement mode, the leakagecurrent between N⁺ sublayer 221 and sublayer 223 of the active strip issuppressed when all control gates on both sides of the active strip areheld at, or below V_(ss), ˜0V. This enhancement threshold voltage can beachieved by providing P⁻ sublayer 222 with a suitable dopantconcentration (e.g., a boron concentration between 1×10¹⁶ and 1×10¹⁷ percm³ or higher, which results in an intrinsic TFT threshold voltage ofbetween ˜0.5 V and ˜1 V).

In some implementations, it may be advantageous to use N⁻ doped orundoped polysilicon or amorphous silicon to implement sublayer 222. Withsuch a doping, some or all of the TFTs along an active string may have anegative threshold voltage (i.e., a depletion mode threshold voltage)and thus require some means to suppress the leakage current. Suchsuppression can be achieved by raising voltage V_(ss) on N⁺ sublayer 221to ˜1V to ˜1.5V and voltage V_(bl) on N⁺ sublayer 223 to a voltage thatis ˜0.5V to ˜2V above that of N⁺ sublayer 221, while holding all localword lines at 0 volt. This set of voltages provides the same effect asholding the word line voltage at ˜−1V to ˜1.5 volts with respect to N⁺sublayer 221 (the source line), and thus suppresses any leakage due toTFTs that are in a slightly depleted threshold voltage. Also, aftererasing the TFTs of a NOR string, the erase operation may require asubsequent soft-programming step that shifts any TFT in the NOR stringthat has been over-erased into a depletion mode threshold voltage backinto an enhancement mode threshold voltage.

Quasi-Volatile Nor Strings

Endurance is a measure of a storage transistor's performance degradationafter some number of write-erase cycles. Endurance of less than around10,000 cycles—i.e., performance being sufficiently degraded as to beunacceptable within 10,000 cycles—is considered too low for some storageapplications requiring frequent data rewrites. However, the NOR stringsof any of the embodiments EMB-1, EMB-2, and EMB-3 of this invention canuse a material for their charge-trapping material 231L and 231R whichprovides a reduced retention times, but which significantly increasestheir endurance (e.g., reducing the retention time from many years tominutes or hours, while increasing the endurance from thousands to tensof millions of write/erase cycles). To achieve this greater endurance,for an ONO film or a similar combination of charge-trapping layers, forexample, the tunnel dielectric layer, typically a silicon oxide film ofthickness 5-10 nm, can be reduced to 3 nm or less, or replacedaltogether with another dielectric film (e.g., silicon nitride or SiN),or can have no dielectric layer at all. Similarly, the charge-trappingmaterial layer may be a CVD-deposited more silicon-rich silicon nitride(e.g., Si_(1.0)N_(1.1)) than conventional Si₃N₄. Under a modest positivecontrol gate programming voltage, electrons will tunnel through thethinner tunnel dielectric by direct tunneling (as distinct fromFowler-Nordheim tunneling, which typically requires higher programmingvoltages) into the silicon nitride charge-trapping material layer wherethe electrons will be temporarily trapped for a period between a fewminutes to a few days. The charge-trapping silicon nitride layer and theblocking layer of silicon oxide (or aluminum oxide or another high-Kdielectric) will keep these electrons from escaping to the word lines,but these electrons will eventually leak back out to sublayers 221, 222,and 223 of the active strip, as electrons are negatively charged andtherefor intrinsically repel each other.

A TFT resulting from these modifications is a low data retention TFT(“semi-volatile TFT” or “quasi-volatile TFT”). Such a TFT may requireperiodic write refreshes or read refreshes to replenish the lost charge.Because the quasi-volatile TFT of the present invention provides aDRAM-like fast read access time with a low latency, the resultingquasi-volatile NOR strings may be suitable for use in some applicationsthat currently require DRAMs. The advantages of quasi-volatile NORstring arrays over DRAMs include: (i) a much lower cost-per-bit figureof merit because DRAMs cannot be readily built in three-dimensionalblocks, and (ii) a much lower power dissipation, as the refresh cyclesneed only be run approximately once every few minutes or once every fewhours, as compared to every ˜64 milliseconds required in current DRAMtechnology.

The quasi-volatile NOR strings of the present invention appropriatelyadapt the program/read/erase conditions to incorporate the periodic datarefreshes. For example, because each quasi-non-volatile NOR string isfrequently read-refreshed or program-refreshed, it is not necessary to“hard-program” quasi-volatile TFTs to open a large threshold voltagewindow between the ‘0’ and ‘1’ states, as compared to non-volatile TFTswhere a minimum 10 years data retention is required. Quasi-non-volatilethreshold voltage window may be as little as 0.2V to 1V, as compared to1V to 3V typical for TFTs that support 10 years' data retention. Thereduced threshold voltage window allows such TFTs to be programmed atlower programming voltages and by shorter-duration programming pulses,which reduce the cumulative electric field stress on the dielectriclayers, thereby extending endurance.

Mirror-Bit Nor Strings

According to another embodiment of the present invention, NOR stringarrays may also be programmed by channel hot-electron injection, similarto that which is used in NROM/Mirror Bit transistors, known to those ofordinary skill in the art. In an NROM/Mirror Bit transistor, chargerepresenting one bit is stored at one end of the channel region next tothe junction with the drain region, and by reversing polarity of thesource and drain, charge representing a second bit is programmed andstored at the opposite end of the channel region next to the sourcejunction. Typical programming voltages are 5 volts at the drainterminal, 0 volt at the source terminal and 8 volts at the control gate.Reading both bits requires reading in reverse order the source and drainjunctions, as is well known to those of ordinary skill in the art.However, channel hot-electron programming is much less efficient thantunnel programming, and therefore channel hot-electron programming doesnot lend itself to the massively parallel programming that is possibleby tunneling. Furthermore, the relatively large programming currentresults in a large IR drop between the N⁺ sublayers (i.e., between thesource and drain regions), thereby limiting the length of the NORstring, unless hard-wire connections are provided to reduce lineresistance, such as shown in FIG. 2b -2 or 2 b-3. Erase operations in aNROM/Mirror Bit embodiment can be achieved using conventional NROM erasemechanism of band-to-band tunneling-induced hot-hole injection. Toneutralize the charge of the trapped electrons, one may apply −5V on theselected word line, 0V on N⁺ sublayer 221 (the source line) and 5V on N⁺sublayer 223 (the drain line). The channel hot-electron injectionapproach doubles NOR string bit-density, making it attractive forapplications such as archival memory.

Embodiments Under a Streamlined Process Flow (“Process Flow A”) forSimultaneous Formation of TFT Channels in Active Strips of MultiplePlanes

The process described above for forming embodiments EMB-1, EMB-2, andEMB-3 can be modified in an alternative but simplified process flow(“Process Flow A”), while improving TFT uniformity and NOR stringperformance across all active strips on multiple planes. In Process FlowA, P⁻ sublayers 222 (i.e., the channels) are simultaneously formed in asingle sequence for all active strips on all planes. This P⁻ channelformation is done late in the manufacturing process flow, after all ormost of the high temperature steps have been completed. Process Flow Ais described below in conjunction with embodiments EMB-1 and EMB-3, butcan be similarly applied to embodiment EMB-2 and other embodiments, andtheir derivatives. In the rest of the detailed description, embodimentsmanufactured under Process Flow A are identified by the suffix “A”appended to their identification. For example, a variation of embodimentEMB-1 manufactured under Process Flow A is identified as embodimentEMB-1A.

FIG. 5a shows a cross section through a Y-Z plane of semiconductorstructure 500, after active layers 502-0 through 502-7 have been formedin a stack of eight planes, one on top of each other, and isolated fromeach other by respective isolation layers 503-0 to 503-7 of material ISLon semiconductor substrate 201. Relative to semiconductor structure 220a of FIG. 2b -1, sublayer 222 of each of active layers 502-0 to 502-7 isformed with, instead of P⁻ polysilicon, sacrificial material SAC1.Isolation layers 503-0 to 503-7, formed with isolation material ISL (adielectric material), separate the active layers on different planes.Sacrificial material SAC1 in sublayers 522-0 to 522-7 will eventually beetched away to make way for P⁻ sublayers. The SAC1 material is selectedsuch that it can be etched rapidly with a high etch selectivity, ascompared to the etch rates of isolation material ISL and N⁺ sublayers523-0 to 523-7, and 521-0 to 521-7. The ISL material may be siliconoxide (e.g., SiO₂), deposited in the thickness range 20-100 nanometer,the N⁺ sublayers may be heavily doped polysilicon, each layer in thethickness range of 20-100 nanometers, and the SAC1 material may be, forexample, one or more of: silicon nitride, porous silicon oxide, andsilicon germanium, in the thickness range 10-100 nanometers. Actualthickness used for each layer is preferably at the lower end of therange to keep to a minimum the total height of the multiple planes,which can be increasingly more difficult to etch anisotropically with32, 64 or more stacked planes.

FIG. 5b is a cross section in a Y-Z plane through buried contacts 205-0and 205-1, through which N⁺ sublayers 523-1 and 523-0 are connected tocircuitry 206-0 and 206-1 in semiconductor substrate 201. Before activelayers 502-0 through 502-7 are formed, buried contacts 205-0 are formedby etching into isolation layer 503-0, so that when N⁺ sublayer 523-0 isdeposited, electrical contact is created with circuitry 206-0 previouslyformed in substrate 201. An optional low resistivity thin metallicsublayer (e.g., TiN and tungsten) of typical thickness range between 5and 20 nm can be deposited (not shown in FIG. 5b ) before N⁺ sublayer523-0 is deposited, so as to lower the line resistance. Low resistivitymetallic plugs such as TiN followed by a thin layer of tungsten can beused to fill the buried contact openings to reduce contact resistance tothe substrate. Active layer 502-0 is then etched into separate blocks,each of which will later be etched into individual active strips. Eachhigher plane of or active layer (e.g., active layer 502-1) extendsbeyond the active layers underneath and has its own buried contacts205-1 connecting it to circuitry 206-1 in substrate 201.

Connecting active strips of each plane to substrate circuitry can beaccomplished either by buried contacts from the bottom (e.g., buriedcontacts 205-0 and 205-1 connecting drain sublayers 523-0 and 523-1 tosubstrate circuitry 206-0 and 206-1 in FIG. 5b ), or by conductor-filledvias from the top of the semiconductor structure (not shown), makingelectrical contacts to N⁺ sublayers 521-0 and 521-1. Because either oneof sublayers 523 and 521 in the same active strip may serve as sourceterminal or drain terminal for the TFTs in the corresponding NOR string,N⁺ sublayers 521 or 523 in the same active strip are interchangeable.The vias are etched through the ISL material in isolation layers 503-0to 503-7 by first forming a stair-stepped multi-plane pyramid-likestructure (i.e., a structure in which the bottom plane extends furthestout), as is well known to a person of ordinary skill familiar with 3D3-DNAND via formation. This alternative contact-from-the-top scheme allowsvias to be etched to reach more than one plane at a time, thus reducingthe number of masking and contact etching steps, which is particularlyuseful when there are 32, 64 or more stacked planes. However, becausesublayers 523 lie underneath of, and are masked by sublayers 521, it isnot easy to contact sublayers 523 using stair-step vias from the top, asthere is a risk that the conductor in the vias may electrically shortsublayers 521 and 523.

According to one embodiment of the present invention, in one process,drain sublayers 523 are connected to the substrate circuitry from thebottom through buried contacts, while the source sublayers 521 areconnected to the substrate circuitry either through hard-wireconnections by conductor-filled vias from the top (e.g., connections 280in FIG. 3a -1). Alternatively, and preferably, the source layers 521 maybe connected to substrate circuitry by the buried contacts using TFTs inthe NOR string that are designated as pre-charge TFTs (i.e., those TFTsthat are used to charge the parasitic capacitance of the NOR string toprovide a virtual voltage source). In this manner, the complexing ofproviding the vias or hard-wire conductors are avoided.

The discussion below focuses on NOR strings in which the source anddrain sublayers connect to substrate circuitry through buried contactsin conjunction with pre-charge TFTs (as described above). Thisarrangement provides the drain and source sublayers appropriate voltagesfor read, program, program-inhibit and erase operations.

Next, all planes may be exposed to a high-temperature rapid thermalannealing and recrystallization step simultaneously applied to N⁺sublayers 521 and 523. This step can also be individually applied toeach plane. Alternatively, rapid thermal annealing, laser annealing forall layers, or shallow laser anneal (e.g., ELA) on one or more planes ata time may also be used. Annealing reduces sheet resistivity of the N⁺sublayers by activating dopants, recrystallization and reducing dopantsegregation at grain boundaries. Of note, because this thermal annealingstep takes place before P⁻ sublayer 522 is formed in any plane, theannealing temperature and duration can be quite high, even in excess of1000° C., which is advantageous for lowering the resistivity of N⁺sublayers 521 and 523.

FIG. 5c is a cross section in the Z-X plane, showing active layers 502-6and 502-7 of structure 500 after trenches 530 along the Y-direction areanisotropically etched through active layers 502-7 to 502-0 to reachdown to landing pads 264 of FIG. 5b . Deep trenches 530 are etched in ananisotropic etch using appropriate chemistry to etch through alternatinglayers of N⁺ material, the SAC1 material, N⁺ material, and the ISLmaterial, to achieve as close as possible vertical trench sidewalls(i.e., achieving substantially the same active strip width and spacingat the top plane and the bottom plane). A hard mask material (e.g.,carbon) may be used during the multi-step etch sequence.

After removing the hard mask residue, trenches 530 are filled with asecond sacrificial material (SAC2) that has different etchcharacteristics from those of the SAC1 material. The SAC2 material maybe, for example, fast etching SiO₂ or doped glass (e.g., BPSG). Like theISL material, the SAC2 material is chosen to resist etching when theSAC1 material is being etched. The SAC2 material mechanically supportsthe tall narrow stacks of active strips, particularly at later stepsthat are performed during and after the SAC1 material is removed, whichleaves cavities between the N⁺ sublayers. Alternatively, such supportcan be provided by local word lines 208W in implementations in which thecharge-trapping material and the local word lines are formed prior toetching the SAC1 material.

Next, narrow openings are masked along the X-direction and etchedanisotropically through the SAC2 material that filled trench 530 to formsecond trenches 545 within the SAC2 material occupying trenches 530, asshown in FIG. 5d . The anisotropic etch exposes vertical sidewalls 547of the active strips throughout the active layers to allow removal ofthe SAC1 material in sublayer 522, thereby forming a cavity between N⁺sublayer 521 and N⁺ sublayer 523 in each active strip of active layers502-0 to 502-7. Secondary trenches 545 allow the formation of aconductive path from the sublayer 522 to the P⁺ substrate region 262-0(labeled V_(bb)) in FIG. 5b . Secondary trenches 545 are preferably each20-100 nanometers wide and may be spaced apart a distance sufficient toaccommodate 64 or more side-by-side local word lines, such as local wordlines 208W-s. Next, a highly selective etch is applied to the exposedsidewalls 547 of FIG. 5d to isotropically etch away all the exposed SAC1material in sublayer 522 through the paths indicated by arrows 547 and548. As discussed above, the SAC1 material can be silicon nitride, whileboth the ISL material and the SAC2 material can be silicon oxide. Withthese materials, hot phosphoric acid may be used to remove the SAC1material, while leaving essentially intact all the N⁺ doped polysiliconin N⁺ sublayers 521 and 523, and the ISL and SAC2 materials in layer 503and trenches 530. Dry-etch processes involving high-selectivitychemistry can achieve a similar result without leaving residues in theelongated cavities previously occupied by the SAC1 material, walledbetween the SAC2 material filling trenches 530.

After the selective removal of the SAC2 material, discussed above, thereare two options in further processing; (i) a first option that firstforms P⁻ sublayers 522 in the cavities between N⁺ sublayers 521 and 523,to be followed by formations of charge-trapping layers and local wordlines 208W; and (ii) a second option that first forms thecharge-trapping layers and local word lines, followed by forming P⁻sublayers 522. The first option is described below in conjunction withFIG. 5e and embodiment EMB-1A of FIG. 5f . The second option isdescribed below in conjunction with embodiment EMB-3A of FIG. 5 g.

FIG. 5e is a cross section through the Z-X plane (e.g., along line 1-1′of FIG. 5d ) away from trench 545, showing active strips in adjacentactive layers supported by the SAC2 material on both sides of eachactive strip. Cavities 537 result from excavating the SAC1 material fromthe space between sublayers 521 and 523 (i.e., the space that isreserved for P⁻ sublayer 522). Optional ultra-thin dopantdiffusion-blocking sublayer 521-d is then deposited on the walls ofcavities 537 (e.g., left wall SOIL, right wall 501R, bottom wall 501B ofN⁺ sublayer 521-7 and top portion 501T of N⁺ drain sublayer 523-7, asshown in FIG. 5e ). Ultrathin dopant diffusion-blocking layer 521-d maybe, for example, silicon nitride, silicon-germanium (SiGe) or othermaterials with atomic lattice smaller than the diameter of the atoms ofthe N⁺ dopant used (e.g., phosphorous, arsenic or antimony) and may bein the thickness range of 0 to 3 nanometers. Dopant diffusion-blockingsublayer 521-d can achieve zero or near zero nanometers thickness by acontrolled deposition of 1-3 atomic layers of the diffusion barriermaterial using, for example, atomic layer deposition (ALD) techniques.Dopant diffusion-blocking layer 521-d may provide the same dopantdiffusion barrier as layers 221-d, 223-d of FIG. 2b-5a , except that,unlike the multiple depositions required of forming layers 221-d and223-d for the multiple active layers, dopant diffusion-blocking layers521-d are formed in a single deposition step for all active layers. Thegaseous material required for the uniform deposition of dopantdiffusion-blocking layer 521-d coats the walls of cavities 537 throughsecondary trenches 545, as shown by arrows 547 and 548 in FIG. 5d . Inno event should the material or thickness of dopant diffusion-blockinglayer 521-d be such that it materially degrades electron conductionacross it, nor should it allow material trapping of electrons as theytunnel through it. If the leakage current between N⁺ sublayers 521 and523 in the active strips is tolerably low, dopant diffusion-blockinglayer 521-d may altogether be omitted.

Next, P⁻ sublayers 522 (e.g., P⁻ sublayer 522-7) are formed along theinside walls 501T, 501B, 501R and SOIL of each cavity, extending alongthe entire length of each active strip. P⁻ sublayers 522 may be dopedpolysilicon, undoped or P-doped amorphous silicon, (e.g., boron-dopedbetween 1×10¹⁶/cm³ and 1×10¹⁸/cm³), silicon-germanium, or any suitablesemiconductor material in a thickness range between 4 and 15 nanometers.In some implementations, P-sublayer 522 is sufficiently thin not tocompletely fill cavities 537, leaving air gap. In other implementations,P⁻ sublayer 522 may be formed sufficiently thick to completely fillcavities 537. After local word lines are formed at a later step, P⁻sublayers 522-6R, and 522-6L (for layer 502-6) along the vertical walls501R, and SOIL serve as the P⁻ channels of TFTs on one or both sideedges of its active strip 550, with N⁺ sublayer 521-6 serving as an N⁺source (at voltage V_(ss)) and N⁺ sublayer 523-6 serving as an N⁺ drain(providing voltage V_(bl)). At a typical thickness of 3-15 nanometers,P⁻ sublayers 522 may be substantially thinner than the width of theircorresponding active strips, which are defined lithographically or maybe defined by spacers well known to a person of ordinary skill in theart. In fact, the thickness of the P⁻ channel formed under this processis independent of the width of the active strips and, even for very thinchannels, P⁻ sublayer 522 has substantially the same thickness in eachof the many active layers. At such reduced thickness, depending on itsdoping concentration, P⁻ sublayers 522-6R and 522-6L are sufficientlythin to be readily completely depleted under appropriate word linevoltages, thereby improving transistor threshold voltage control andreducing leakage between the N⁺ source and drain sublayers along theactive strip.

Simultaneously, P-doped polysilicon is deposited along the verticalwalls of secondary trenches 545 to form pillars 290 (not shown in FIG.5e , but shown as pillars 290 in FIG. 5f ) extending from the top planeto the bottom plane. At the bottom plane, connections are made betweenpillars 290 and circuitry in substrate 201 (e.g., voltage sourceproviding voltage V_(bb)). If dopant diffusion-blocking sublayer 521-dis provided, prior to forming P⁻ sublayer 522 and pillars 290, a briefanisotropic etch may be needed to etch away layer 521-d at the bottom oftrench 545 to allow direct contact between the P⁻ doped pillars 290 andthe P⁺ circuitry that provides back-bias V_(bb) and erase voltageV_(erase) from substrate 201 (e.g., circuitry 262-0 in FIG. 5b ).Pillars 290 are spaced apart along the length of each active strip toaccommodate the formation (in a subsequent step) of 32, 64, 128 or morevertical local word lines 208W in-between the pillars (see, FIG. 5f ) ofembodiment EMB-1A. (This separation is set by the separation ofsecondary trenches 545.)

Pillars 290 connect P⁻ sublayers 222 (e.g., P⁻ sublayers 522-6R and522-6L) of all the active layers—which serve as channel regions of theTFTs—to circuitry in substrate 201, so as to provide P⁻ sublayers 222with an appropriate back-bias voltage. Circuitry in the substrate istypically shared by TFTs of all active strips in semiconductor structure500. Pillars 290 provide back-bias voltage V_(bb) during read operationsand high voltage V_(erase), typically 10V to 20V, during block-eraseoperations. However in some implementations (see below, and FIGS. 6a-6c), an erase operation can be accomplished without the use of asubstrate-generated voltage, in which case pillar 290 connections to P⁺circuitry (e.g., P⁺ circuitry 262-0) may not be needed, so that the thinpolysilicon along the vertical walls of the pillars 290 may be etchedaway (being careful to not etch away the channel region P⁻ sublayers 522(e.g., P⁺ sublayers 522-6R, and 522-6L of FIG. 5e , inside the cavitiesbordered by walls 501B, 501T, 501R and SOIL).

In the next step, the SAC2 material remaining in trenches 530 areremoved using, for example, a high selectivity anisotropic etch whichexposes the side-walls of all active strips except where thespaced-apart pillars 290 are located. Next, charge-trapping layers 231Land 231R are deposited conformally on the exposed sidewalls of theactive strips. FIG. 5f illustrates, in a cross section in the X-Y planeof embodiment EMB-1A of the present invention, P-doped pillars 290,local word lines 280W and pre-charge word lines 208-CHG are provided inadjacent active strips of active layer 502-7, after suitable masking,etching and deposition steps.

The remaining process steps follow the corresponding steps in formingembodiments EMB-1, EMB-2 and EMB-3 as previously discussed, asappropriate. Before forming charge-trapping layers 531, the exposed sideedges of optional ultrathin dopant diffusion-blocking layer 521-d may beremoved by a short isotropic etch, followed by forming charge-trappinglayers 531 on one or both exposed sidewalls of the active layers,followed by forming local word lines 208W along both side edges (e.g.,embodiment EMB-1A of FIG. 5f ). Alternatively, the ultrathin dopantdiffusion-blocking layers 521-d at the exposed side edges of thecavities are oxidized to form part or all the thickness of the tunneldielectric layer over P⁻ sublayer 522, while at the same time formingthicker tunnel dielectric layer over the exposed side edges of N⁺sublayers 521 and 523. The thicker tunnel dielectric layer is around 1to 5 nanometers thicker than the tunnel dielectric layer over P⁻sublayer 522 because the oxidation rate of N⁺ doped polysilicon isconsiderably faster than the oxidation rate of silicon nitride. AsFowler-Nordheim tunneling current is exponentially dependent on thetunneling dielectric thickness, even a 1 nanometer thicker tunnel oxidelayer significantly impedes charge tunneling from the N⁺ regions intocharge-trapping layer 531 during programming

FIG. 5g shows a cross section in the Z-X plane of active layers 502-6and 502-7 of embodiment EMB-3A formed using the process of the secondoption. FIG. 5g shows embodiment EMB-3A after formation of optionalultra-thin dopant diffusion-blocking layer 521-d and deposition ofundoped or P⁻ doped polysilicon, amorphous silicon or silicon germaniumin sublayer 522 that forms the channel regions of TFTs T_(R) 585, T_(R)587. The channel material is also deposited on side walls of trenches545 to form pillars 290 for connecting the channel regions of the TFTs(i.e., P⁻ sublayer 522) to substrate circuitry 262. The simultaneouslyformed P⁻ sublayers 522 in all active layers provide a channel length L.Cavity 537 and gap 538 between neighboring pillars 290 can be filledcompletely with a thicker P⁻ polysilicon or silicon germanium, left aspartial air-gap isolation, or filled with dielectric isolation (e.g.,silicon dioxide). Pillars 290 surrounding the sides of active strips502-6, and 502-7 in embodiment EMB-3A provide desirable electricalshielding to reduce the parasitic capacitive coupling between adjacentactive strips on the same plane. Capacitive shielding between activestrips on adjacent planes in a stack can be enhanced by etching the ISLmaterial in the isolation layers (e.g., isolation layers 503-6 and503-7) in part or in whole (not shown in FIG. 5g ).

Under the second option process, i.e., forming charge-trapping layer 531before the P⁻ sublayer 522, the ISL material between the active layerscan be etched (prior to removal of the SAC1 material) to expose the backside of charge-trapping layer 531. The exposed back side ofcharge-trapping layer 531 allows tunnel dielectric (typically, SiO₂) andpart or all of the exposed charge-trapping material (typicallysilicon-rich silicon nitride), as indicated in FIG. 5g by area 532X, tobe removed. Shaded area 532X interrupts the path by which electrons thatare trapped over TFT channels (i.e., the region indicated by L) may belost through sideways hopping conduction in the silicon-rich siliconnitride layer along arrow 577. The cavity left in area 532 x after theISL material and the exposed charge-trapping material are removed can befilled with another dielectric layer following removal of the SAC1material from sublayer 522 or be left as an air gap. In embodimentswhere the ISL material is only partially removed, pillars 290 can fillup the etched ISL resulting spaces to partially isolate N⁺ sublayer 523of TFT T_(R) 585 from N⁺ sublayer 521 of TFT T_(R) 587. As in embodimentEMB-1A, all P-sublayers 522 in the active layers are connected viapillars 290 to P⁺ circuitry 262-0 in substrate 201.

Dopant diffusion-blocking film 521-d can be formed (FIG. 5g ) in asingle step for all active layers prior to deposition of P⁻ sublayers522, thus greatly simplifying the repetitive process of FIG. 2b -5.However, because deposition of P⁻ sublayers 522 is performed almost atthe end of the process, after all high-temperature anneals have alreadytaken place, ultra-thin dopant diffusion-blocking layer 521-d may beomitted. In embodiments in which connections of pillars 290 to substratecircuitry is are not needed for erase operations, the vertical walls ofP⁻ pillars 290 that are within trenches 530 may be etched away, leavingonly P⁻ sublayers 522 lining the cavities 537 (FIG. 5g ) and leavingtrenches 530 as air-gap isolation between adjacent active strips of allplanes.

Pillars 290 and conductors 208W provide electrical shielding to suppressthe parasitic capacitive coupling between adjacent thin film transistorsof each plane. As seen from FIG. 5g , pillars 290 and P⁻ sublayers 522may be formed prior to or following formations of charge-trappingmaterial 531 and local word line 208W.

The process sequences presented above are by way of examples, it beingunderstood that other process sequences or deviations may also be usedwithin the scope of the present invention. For example, instead of fullyexcavating the SAC1 material to form the cavities for subsequentlyforming sublayers 522, an alternative approach is to selectively etchthe SAC1 material in a controlled sideway etch to form recesses inwardfrom one or both side edges of the stack, leaving a narrowed-down spineof the SAC1 material that mechanically supports the separation betweenN⁺ sublayers 523 and N⁺ sublayers 521, then simultaneously filling allplanes with the channel material in first sublayer 522, followed byremoving the channel material from the sidewalls of trenches 530,resulting in P⁻ sublayers 522-0 to 522-7 residing in the recesses thatare now isolated from each other by the remaining spine of the SAC1material, followed by the next process steps to form charge-trappingmaterial 531 and conductors 208W. These steps are illustrated in FIG. 5h-1 through FIG. 5h -3. Specifically, FIG. 5h -1 shows cross section 500in the Z-X plane, showing active strips immediately prior to etching thesacrificial SAC1 material between N⁺ sublayers 521 and 522, inaccordance with one embodiment of the present invention. FIG. 5h -2shows cross section 500 of FIG. 5h -1, after sideway selective etchingof the SAC1 material (along the direction indicated by reference numeral537) to form selective support spines out of the SAC1 material (e.g.,spine SAC1-a), followed by filling the recesses with P⁻ doped channelmaterial (e.g., polysilicon) and over the sidewalls of the activestrips, according to one embodiment of the present invention. FIG. 5h -3shows cross section 500 of FIG. 5h -2, after removal of the P⁻ materialfrom areas 525 along the sidewalls of the active strips, while leavingP⁻ sublayer 522 in the recesses, in accordance with one embodiment ofthe present invention. FIG. 5h -3 also shows removal of isolationmaterials from trenches 530, formation of charge-trapping layer 531 andlocal word lines 208-W, thereby forming transistors 11585 and T_(R) 585on opposite sides of the active strips.

In FIGS. 5a, 5b and 5c , N⁺ sublayers 521-0 to 521-7 and 523-0 to 523-7can all be formed in a single deposition step under another process(“Process Flow B”). Under Process Flow B, third sacrificial layer (adielectric material SAC3, not shown) may be deposited in place of N⁺sublayers 521 and 523. Then, similar to the way the SAC1 material wasetched to form cavities to be filled by P⁻ polysilicon, the SAC3material may be etched away to form cavities to be filled by N⁺ dopedpolysilicon simultaneously for all planes in semiconductor 500. The SAC3material should have a high etch selectivity to the ISL, SAC1 and SAC2materials already in place. An anisotropic etch (ending with a briefisotropic etch to remove thin polysilicon stringers) to remove the N⁺polysilicon in trenches 530 that would otherwise be shorting verticallyadjacent N⁺ source and N⁺ drain sublayers. Under Process Flow B, theSAC3 material from all sublayers 521 and 523 of active layers arepreferably etched simultaneously to cavities and then filled by N⁺polysilicon, so that all N⁺ sublayers 521 and 523 can be annealed in asingle high-temperature rapid anneal step. Only after the anneal step,cavities 537 (FIGS. 5e and 5g ) are formed by etching the SAC1 materialand then filling the resulting cavities with P-polysilicon to form P⁻sublayer 522. Under Process Flow B, all active layers 502-0 to 502-7 maypreferably be connected to the substrate circuitry 206-0 and 206-1 fromthe top of semiconductor structure 500 through a “stair-step via”scheme, instead of the buried contacts 205-0, 205-1 of FIG. 5 b.

Source-Drain Leakage in Long Nor Strings

In long NOR strings, the current of the one accessed TFT in a readoperation has to compete with the cumulative subthreshold leakagecurrents from the thousand or more parallel unselected TFTs. Similarly,pre-charged strip capacitor C has to contend with charge leakage notjust of one transistor (as in a DRAM circuit) but the charge leakagethrough the thousand or more transistors in the NOR string. That chargeleakage reduces substantially the charge retention time on C to perhapsa few hundred microseconds, requiring counter measures to reduce orneutralize such leakage, as discussed below. However, as will bediscussed below, the leakage for a thousand or so transistors only comesinto play during read operations. During program, program-inhibit orerase operations, source sublayer 221 and bit line sublayer 223 arepreferably held at the same voltage, therefore transistor leakagebetween the two sublayers is insignificant (the leakage of charge fromcapacitor C during program, program-inhibit or erase operations isprimarily to the substrate through the substrate selection circuitry,which is formed in single-crystal or epitaxial silicon where transistorleakage is very small). For a read operation, even a relatively short100-microsecond retention time of charge on the source and bit linecapacitors is ample time to complete the sub-100 nanosecond readoperation (see below) of the TFTs of the present invention. A keydifference between a TFT in a NOR string of the present invention and aDRAM cell is that the former is a non-volatile memory transistor, sothat even if parasitic capacitor C is completely discharged theinformation stored in the selected TFT is not lost from the chargestorage material (i.e., charge-trapping layers 231 in embodiments EMB-1,EMB-2 and EMB-3), unlike a DRAM cell where it is forever lost unlessrefreshed. Capacitor C is used solely to temporarily hold the pre-chargevoltage on N⁺ sublayers 221 and 223 at one of voltages V_(ss), V_(bl),V_(progr), V_(inhibit), or V_(erase); C is not used to store actual datafor any of the non-volatile TFTs in the string. Pre-charge transistor303, controlled by word line 151 n (208-CHG) (FIG. 3a -1) is activatedmomentarily immediately preceding read, program, program-inhibit orerase operations to transfer through connections 270 the voltage V_(bl)from the substrate circuitry (not shown) to capacitor C of sublayer 221.For example, voltage V_(bl) can be set at ˜0V to pre-charge N⁺ sublayer221 to a virtual ground voltage ˜0V during read, or to pre-charge bothN⁺ sublayers 221 and 223 to between ˜5V and ˜10V during program inhibit.The value of cumulative capacitors C may be increased by lengthening theactive string to accommodate thousands more TFTs along each side of thestring, correspondingly increasing the retention time of pre-chargevoltage V_(ss) on N⁺ sublayer 221. However, a longer NOR string suffersfrom an increased resistance R as well as higher leakage current betweenN⁺ sublayer 221 and N⁺ sublayer 223; such leakage current may interferewith the sensed current when reading the one TFT being addressed withall other TFT's in their “off” (but somewhat leaky) state. To speed upthe pre-charging of the capacitance C of a long active strip, severalpre-charge TFTs 303 may be provided spaced apart along either side ofthe active strip (e.g., once every 128, 256 or more TFTs).

Non-Volatile Memory TFTs with Highly Scaled Short Channels

Ultra-thin diffusion-blocking layer 521-d enables a highly scaledchannel length in non-volatile memory TFTs (“ultra-short channel TFTs”;e.g., the channel length L in TFT T_(R) 585 of FIG. 5f ) by reducing thethickness of the SAC1 material. For example, the highly scaled channellength may be 40 nanometers or less, while the thickness of the SAC1material standing in place for P⁻ sublayer 522 may be reduced to 20nanometers or less. TFT channel scaling is enhanced by having extremelythin P⁻ sublayer 522, in the range of 3-10 nanometers, sufficient tosupport the TFT channel inversion layer but thin enough to be depletedthrough its entire depth under appropriate control gate voltage. A readoperation for an ultra-short channel TFT requires P⁻ sublayer 522 to berelatively heavily P⁻ doped (e.g., between 1×10¹⁷/cm³ and 1×10¹⁸/cm³). Ashorter channel length results in a higher read current at a lower drainvoltage, thus reducing power dissipation for read operations. A highlyscaled channel has the added benefit of a lesser total thickness in theactive layers, thus making the easier to etch from the top active layerto the bottom active layer. Ultra-short channel TFTs also can be erasedthrough a lateral-field-assisted charge-hopping and tunnel-erasemechanism, which is discussed below in conjunction with FIG. 7.

Exemplary operations for the NOR strings of the present invention aredescribed next.

Read Operations.

To read any one TFT among the many TFTs along a NOR string, the TFTs onboth sides of an active strip are initially set to a non-conducting or“off” state, so that all global and local word lines in a selected blockare initially held at 0 volts. As shown in FIG. 3a , the addressed NORstring (e.g. NOR string 202-1) can either share a sensing circuit amongseveral NOR strings through a decoding circuitry in substrate 201, oreach NOR string may be directly connected to a dedicated sensingcircuit, so that many other addressed NOR strings sharing the same planecan be sensed in parallel. Each addressed NOR string has its source line(i.e., N⁺ sublayer 221) initially set at Vs, ˜0V. (To simplify thisdiscussion, in the context of FIGS. 3a -1 to 3 c, the N⁺ sublayers 221and 223 are referred to as source line 221 and bit line or drain line223, respectively). In an implementation using a hard-wired sourceconnection, voltage V_(ss) is supplied from substrate 201 to source line221 through hard-wired connections 280. FIG. 3b illustrates a typicalread cycle for a NOR string with hard-wired source voltage V_(ss).Initially, all word lines are at 0V and the voltage on source line 221is held at 0V through connections 280. The voltage on bit line 223 isthen raised to V_(bl)˜0.5 V to 2V, supplied through connections 270 fromthe substrate, and is also the voltage at an input to a sense amplifier(V_(SA)). After bit line 223 is raised to VIII, the selected word line(word line 151 a; labeled “WL-sel”) is ramped up (shown in FIG. 3b asincremental stepped voltages) while all other non-selected word lines(word line 151 b; labeled “WL-nsel”) remain in their “off” state (0V).When the voltage on the selected gate electrode exceeds the thresholdvoltage programmed into the selected TFT (e.g. transistor 152-1 on strip202-1) it begins conducting, and thus begins to discharge voltage V_(bl)(event A in FIG. 3b ) which is detected by the sense amplifier connectedto addressed string 202-1.

In embodiments EMB-1, EMB-2 and EMB-3 employing pre-charging ofparasitic cumulative capacitance C (i.e., the total capacitance of allcapacitors labeled 360 in each NOR string in FIG. 3a -1) to a “virtualV_(ss)” voltage, pre-charge TFT 303 (FIG. 3b ) shares source line 221and bit line or drain line 223 of the NOR string (pre-charge TFT 303 mayhave the same construction as the memory TFTs, but is not used as amemory transistor and may have a wider channel to provide a greatercurrent during the pre-charge pulse) and has its drain line 223connected through connections 270 to bit line voltage V_(bl) insubstrate 201. In a typical pre-charge/read cycle (see FIG. 3c ) V_(bl)is initially set at 0V. Pre-charge word line 208-CHG of TFT 303 ismomentarily raised to around 3V to transfer V_(bl)˜0V from bit line 223to source line 221 to establish a “virtual V_(ss)” voltage ˜0V on sourceline 221. Following the pre-charge pulse, bit line 223 is set to aroundV_(bl)˜2V through bit line connection 270. The V_(bl) voltage is alsothe voltage at the sense amplifier for the addressed NOR string. The oneselected global word line and all its associated vertical local wordlines 151 a (labeled “WL-sel”) (i.e. slice 114 of FIG. 1a -2) are rampedfrom 0V to typically 3V-4V (shown as stepped voltages in FIG. 3d ) orhigher if a larger window of operation is desired between the erased andprogrammed V_(th) voltages, while all other global word lines and theirlocal word lines in the block are in their “off” state (0V). If theselected TFT is in an erased state (i.e., V_(th)=V_(erase)˜1 volt), bitline voltage V_(bl) will begin to discharge toward source voltage V_(ss)when its word line voltage rises above ˜1V. If the selected TFT has beenprogrammed to V_(th)˜2V, the bit line voltage will begin dischargingonly when its word line rises above ˜2V. A voltage dip in voltage V_(bl)(event B in FIG. 3c ) is detected at the sense amplifier when the chargestored on bit line 223 begins to discharge through the selected TFTtowards voltage V_(ss) on source line 221. All non-selected word lines151 b (labeled “WL-nsel”) in the NOR string are “off” at 0V, even thoughthey may each contribute a sub-threshold leakage current between N⁺sublayer 223 and N⁺ sublayer 221. Accordingly, it is important that theread operation follows closely the pre-charge pulse before this leakagecurrent begins to seriously degrade the V_(ss) charge on capacitors C ofthe NOR string. The pre-charge phase typically has a duration between 1and 10 nanoseconds, depending on the magnitude of distributedcapacitance C and distributed resistance R of N⁺ sublayers 221 and 223,and the pre-charge current supplied through pre-charge TFTs 303. Thepre-charge can be sped up by augmenting the current through pre-chargeTFTs 303 using some of the memory TFTs along the NOR string to servetemporarily as pre-charge transistors, although care must be taken toavoid driving their gate voltages high enough during the pre-chargepulse as to cause a disturb condition on their programmed thresholdvoltage.

All TFTs 152-0 to 152-3 within slice 114 (FIG. 1a -2) experience thesame ramping voltage on their local word line 151 a (WL-sel), andtherefore TFTs on different active strips on different planes can beread simultaneously (i.e., in parallel) during a single read operation,provided that the active strips on different active layers 202-0 to202-7 are all pre-charged (either individually or at the same time) whenthe read operation begins from their respective substrate circuitrythrough their pre-charge TFTs 303, and provided that the active stripson the different active layers have dedicated sense amplifiers connectedthrough individual connections 270. This slice-oriented read operationincreases the read bandwidth by a factor corresponding to the number ofplanes in memory block 100.

Multibit (MLC), Archival, and Analog Thin-Film Transistor Strings

In an embodiment where MLC is used (i.e., Multi-Level cell, in whichmore than one bit of information is stored in a TFT), the addressed TFTin a NOR string may be programmed to any of several threshold voltages(e.g., 1V (for an erased state), 2V, 3V or 4V, for the four statesrepresenting two bits of data). The addressed global word line and itslocal word lines can be raised in incremental voltage steps untilconduction in the selected TFTs is detected by the respective senseamplifiers. Alternatively, a single word line voltage can be applied(e.g., ˜5V), and the rate of discharge of voltage V_(bl) can be comparedwith the rate of discharge of each of several programmable referencevoltages representative of the four voltage states of the two binarybits stored on the TFT. This approach can be extended to store eightstates (for 3-bit MLC TFTs), sixteen states or a continuum of states,which effectively provides analog storage. The programmable referencevoltages are stored on reference NOR strings, typically in the sameblock, preferably located in the same plane as the selected NOR stringto best track manufacturing variations among active strips on differentplanes. For MLC applications, more than one programmable reference NORstring may be provided to detect each of the programmed states. Forexample, if 2-bit MLC is used, three reference NOR strings, one for eachintermediate programmable threshold voltage (e.g. 1.5V, 2.5V, 3.5 V inthe example above) may be used. Since there may be thousands of activestrips on each plane in a block, the programmable reference NOR stringscan be repeated, for example, with one set shared between every 8 ormore NOR strings in a block.

Alternatively, the reference NOR string can be programmed to a firstthreshold voltage (e.g., ˜1.5V that is slightly above the erased voltageof ˜1V), so that the additional ˜2.5V and ˜3.5 V reference programmedvoltage levels can be achieved by pre-charging the virtual sourcevoltage V_(ss) (source line 221) of the reference NOR string with astepped or ramped voltage starting from ˜0V and raising it to ˜4V, whilecorrespondingly increasing the voltage V_(bl) on the reference NORstring bit line 223 to be ˜0.5 V higher than the V_(ss) voltage. All thewhile the word line voltage applied to the reference TFT and the wordline voltage applied to the memory TFT being read are the same, as theyboth are driven by the same global word line. This “on the fly” settingof the various reference voltages is made possible because eachreference NOR string can be readily set to its individual gate-sourcevoltage, independent of all other NOR strings in the block.

The flexibility for setting the reference voltages on a reference NORstring by adjusting its V_(ss) and V_(bl) voltages, rather than byactually programming the reference TFT to one or another of the distinctthreshold voltages, enables storing of a continuum of voltages,providing analog storage on each storage TFT of a NOR string. As anexample, during programming, the reference NOR string can be set to atarget threshold voltage of 2.2V, when programming the storage TFT to˜2.2 V. Then during reading the reference string's voltages V_(ss) andV_(bl) are ramped in a sweep starting at ˜0V and ending at ˜4V, with theword lines for both the reference TFT and the storage TFT at ˜4V. Solong as the ramping reference voltage is below 2.2V, the signal from thereference TFT is stronger than that of the programmed memory TFT. Whenthe reference TFT has ramped past 2.2V, the signal from the referenceTFT becomes weaker than the signal from the storage TFT, resulting inthe flipping of the output signal polarity from the differential senseamplifier, indicating 2.2V as the stored value of the programmed TFT.

The NOR strings of the present invention can be employed for archivalstorage for data that changes rarely. Archival storage requires thelowest cost-per-bit possible, therefore selected archival blocks of theNOR string of the current invention can be programmed to store, forexample, 1.5, 2, 3, 4 or more bits per TFT. For example, storing 4 bitsper TFT requires 16 programmed voltages between ˜0.5V and ˜4V. Thecorresponding TFT in the reference NOR string can be programmed at˜0.5V, while programming the storage TFT to the target threshold. Duringa read operation, the reference string's source and drain voltagesV_(ss) and V_(bl) are stepped up in ˜0.25V increments until the outputpolarity of the sense amplifier flips, which occurs when the signal fromthe reference NOR string becomes weaker than the signal from the storageor programmed TFT. Strong ECC at the system controller can correct anyof the intermediate programmed states that have drifted during longstorage or after extensive number of reads.

When the NOR strings in a block suffer from excessive source to drainleakage even when all TFTs of the NOR string are turned off, suchleakage can be substantially neutralized by designation leakagereference strings in which the leakage current of the reference stringis modulated by adjusting the voltages on its shared source V_(ss) andshared drain V_(bl) until its leakage substantially matches the leakagecurrents of the non-reference NOR strings in the same block.

Revolving Reference Nor String Address Locations to Extend CycleEndurance.

In applications requiring a large number of write/erase operations, thethreshold-voltage window of operation for the TFTs in the NOR stringsmay drift with cycling, away from the threshold-voltage window that isprogrammed into the TFTs of the reference NOR strings at the device'sbeginning of life. The growing discrepancy between TFTs on the referenceNOR strings and TFTs on the addressed memory NOR strings over time, ifleft unattended, can defeat the purpose of having reference NOR strings.To overcome this drift, reference NOR strings in a block need not alwaysbe at the same physical address, and need not be permanently programmedfor the entire life of the device. Since the programmable reference NORstrings are practically identical to the memory NOR strings sharing thesame plane in a block, reference NOR strings need not be dedicated forthat purpose in any memory array block. In fact, any one of the memoryNOR strings can be set aside as a programmable reference NOR string. Infact, the physical address locations of the programmable reference NORstrings can be rotated periodically (e.g. changed once every 100 timesthe block is erased) among the sea of memory NOR strings, so as to levelout the performance degradation of memory NOR strings and reference NORstrings as a result of extensive program/erase cycles.

According to the current invention, any NOR string can be rotatedperiodically to be designated as a programmable reference NOR string,and its address location may be stored inside or outside the addressedblock. The stored address may be retrieved by the system controller whenreading the NOR string. Under this scheme, rotation of reference NORstrings can be done either randomly (e.g., using a random numbergenerator to designate new addresses), or systematically among any ofthe active memory NOR strings. Programming of newly designated referenceNOR strings can be done as part of the erase sequence when all TFTs on aslice or a block are erased together, to be followed by setting anew thereference voltages on the newly designated set of reference NOR strings.In this manner, all active memory NOR strings and all reference NORstrings in a block drift statistically more or less in tandem throughextensive cycling.

Programmable Reference Slices.

In some embodiments of the present invention, a block may be partitionedinto four equal-size quadrants, as illustrated in FIG. 6a . FIG. 6a showsemiconductor structure 600, which is a three-dimensional representationof a memory array organized into quadrants Q1-Q4. In each quadrant, (i)numerous NOR strings are each formed in active strips extending alongthe Y-direction (e.g., NOR string 112), (ii) pages extending along theX-direction (e.g., page 113), each page consisting of one TFT from eachNOR string at a corresponding Y-position, the NOR strings in the pagebeing of the same corresponding Z-position (i.e., of the same activelayer); (iii) slices extending in both the X- and Z-directions (e.g.,slice 114), with each slice consisting of the pages of the samecorresponding Y-position, one page from each of the planes, and (iv)planes extending along both the X- and Y-directions (e.g., plane 110),each plane consisting of all pages at a given Z-position (i.e., of thesame active layer).

FIG. 6b shows structure 600 of FIG. 6a , showing TFTs in programmablereference NOR string 112-Ref in quadrant Q4 and TFTs in NOR string 112in quadrant Q2 coupled to sense amplifiers SA(a), Q2 and Q4 being“mirror image quadrants.” FIG. 6b also shows (i) programmable referenceslice 114-Ref (indicated by area B) in quadrant Q3 similarly providingcorresponding reference TFTs for slice 114 in mirror image quadrant Q1,sharing sense amplifiers SA(b), and (ii) programmable reference plane110-Ref in quadrant Q2 providing corresponding reference TFTs to plane110 in mirror image quadrant Q1, sharing sense amplifiers SA(c), andalso providing corresponding reference TFTs for NOR strings in the samequadrant (e.g., NOR string 112).

As shown in FIG. 6b , programmable reference NOR strings 112 Ref may beprovided in each quadrant to provide reference voltages for the memoryNOR strings on the same plane in the same quadrant, in the manneralready discussed above. Alternatively, programmable reference slices(e.g., reference slice 114 Ref) are provided on mirror-image quadrantsfor corresponding memory slices. For example, when reading a memoryslice in quadrant Q1, programmed reference slice 114 Ref (area B) inquadrant Q3 is simultaneously presented to sense amplifiers 206 that areshared between quadrants Q1 and Q3. Similarly, when reading a memoryslice in quadrant Q3, reference slice 114 Ref (area A) of quadrant Q1 ispresented to the shared sense amplifiers 206. There can be more than onereference slice distributed along the length of NOR strings 112 topartially accommodate mismatched in RC delay between the slice beingread and its reference slice. Alternatively, the system controller cancalculate and apply a time delay between the global word line of theaddressed slice and that of the reference slice, based on theirrespective physical locations along their respective NOR strings. Wherethe number of planes is a high number (e.g. 8 or more planes), one ormore planes can be added at the top of the block to serve either as aredundant plane (i.e., to substitute for any defective plane) in thequadrant, or as programmable reference pages, providing referencethreshold voltages for the addressed pages sharing the same global wordline conductor 208 g-a. The sense amplifier at the end of each NORstring receives the read signal from the addressed page at the same timeas it receives the signal from the reference page at the top of theblock, since both pages are activated by the same global word line.

In one embodiment, each memory block consists of two halves, e.g.,quadrants Q1 and Q2 constitute an “upper half” and quadrants Q3 and Q4constitute a “lower half.” In this example, each quadrant has 16 planes,4096 (4K) NOR strings in each plane, and 1024 (1K) TFTs in each NORstring. It is customary to use the unit “K” which is 1024. Adjacentquadrants Q1 and Q2 share 1K global word lines (e.g., global word line208 g-a) driving 2048 (2K) local word lines 208W per quadrant (i.e., onelocal word line for each pair of TFTs from two adjacent NOR strings). 4KTFTs from quadrant Q1 and 4K TFTs from quadrant Q2 form an 8K-bit pageof TFTs. 16 pages form a 128K-bit slice, and 1K slices are provided in ahalf-block, thus providing 256 Mbits of total storage per block. (Here,1 Mbits is 1K×1 Kbits.) The 4K strings in each plane of quadrants Q2 andQ4 share substrate circuitry 206, including voltage sources for voltageV_(bl) and sense amplifiers (SA). Also included in each quadrant areredundant NOR strings that are used as spares to replace faulty NORstrings, as well to store quadrant parameters such as program/erasecycle count, quadrant defect map and quadrant ECC. Such system data areaccessible to a system controller. For blocks with high plane counts, itmay be desirable to add one or more planes to each block as spares forreplacing a defective plane.

Programmable Reference Planes, Spare Planes

High capacity storage systems based on arrays of the NOR strings of thepresent invention require a dedicated intelligent high-speed systemcontroller to manage the full potential for error-free massivelyparallel erase, program and program-inhibit, and read operations thatmay span thousands of “chips” including millions of memory blocks. Toachieve the requisite high speed, off-chip system controllers typicallyrely on state machines or dedicated logic functions implemented in thememory circuits. As well, each memory circuit stores system parametersand information related to the files stored in the memory circuit. Suchsystem information is typically accessible to the system controller, butnot accessible by the user. It is advantageous for the system controllerto quickly read the memory circuit-related information. For a binarymemory system in which 1 bit is stored per TFT (e.g., in the blockorganization of FIG. 6a ), the storage capacity in each block accessibleto the user is given by 4 quadrants×16 planes per block×4K NOR stringsper plane per quadrant×1K TFTs per NOR string, which equals 256M bits.

A block under this organization (i.e., 256 Megabits) provides 2K slices.A terabit memory circuit may be provided by including 4K blocks.

As shown in FIGS. 6a and 6b , the TFTs in quadrants Q2 and Q4 sharevoltage source V_(bl), sense amplifiers SA, data registers, XOR gatesand input/output (I/O) terminals to and from substrate circuitry 206.According to one organization, FIG. 6a shows NOR strings 112,quarter-planes 110, half-slices 114, and half-pages 113. Also shown arepillars 290 supplying back-bias voltage V_(bb) from the substrate. FIG.6b shows examples of locations of reference strings 112 (Ref), referenceslices 114 (Ref) and reference planes 110 (Ref). In the case ofreference strings, reference string 112 (Ref) of quadrant Q4 can serveas a reference string to NOR string 112 on the same plane in quadrantQ2, the two NOR strings being presented to a shared differential senseamplifier SA in circuitry 206. Similarly, reference slice 114 Ref (areaA) in quadrant Q1 can serve as reference for a slice in quadrant Q3,while a reference slice B in quadrant Q1 can serve as reference forslices in quadrant Q3, again sharing differential sense amplifiers SAprovided between quadrants Q1 and Q3. Global word lines 208 g-a areconnected to local word lines 208W and local pre-charge word lines208-CHG. Substrate circuitry and input/output channels 206 are sharedbetween TFTs in quadrants Q2 and Q4. Under this arrangement, theirphysical locations allow cutting by half the resistance and capacitanceof NOR strings 112. Similarly, global word line drivers 262 are sharedbetween quadrants Q1 and Q2 to cut by half the resistance andcapacitance of the global word lines, and pillars 290 (optional) connectP⁻ sublayers of NOR strings 112 to the substrate voltage.

Since silicon real estate on an integrated circuit is costly, ratherthan adding reference strings or reference pages to each plane, it maybe advantageous to have some or all reference strings or reference pagesprovided in one or more additional planes. The additional plane orplanes consume minimal additional silicon real-estate and the referenceplane has the advantage that the addressed global word line 208 g-aaccesses a reference page at the same time it accesses an addressed pageon any of the planes at the same address location along the activestrings in the same quadrant. For example, in FIG. 6b , reference string112 Ref, which is shown as dashed line in quadrant Q2, resides inreference plane 110 Ref in this example. NOR string 112 Ref tracksmemory NOR string 112 being selected for read in the same quadrant andthe read signals from the two NOR strings reach the differential senseamplifiers SA for that quadrant practically at the same time. Althoughreference plane 110 Ref is shown in FIG. 6b as being provided in the topplane, any plane in the quadrant can be designated a reference plane. Infact, it is not be necessary for every NOR string on the reference planeto be a reference string: e.g., every one in eight NOR strings can bedesignated as a reference NOR string that is shared by eight NOR stringsin other planes. The remainder of NOR strings in the reference plane mayserve as spare strings to substitute for defective strings on the otherplanes in the block.

Alternatively, one or more additional planes (e.g., plane 117 in FIG. 6c) can be set aside to serve as spare memory resources to substitute fordefective NOR strings, defective pages or defective planes in the samequadrant.

As related to electrically programmable reference strings, slices, pagesor planes, once set in their designated threshold voltage states, caremust be exercised at all times to inhibit their inadvertent programmingor erasing during programming, erasing or reading the non-referencestrings.

A very large storage system of 1 petabyte (8×10¹⁵ bits) requires 8,0001-terabit memory circuits (“chips”), involving 32M blocks or 64 Gslices. (1 Gbits is 1K×1 Mbits). This is a large amount of data to bewritten (i.e. programmed) or read. Therefore, it is advantageous to beable to program and read in parallel a great many blocks, slices orpages on numerous chips at once, and to do so with minimum powerdissipation at the system level. It is also advantageous for a terabitcapacity memory chip to have many input/output channels such thatrequested data can be streamed in and out in parallel from and to alarge number of blocks. The time required to track down the physicallocation of the most current version of any given stored file or dataset would require a significant amount of time for the system controllerto maintain, such as the translation the logical address into the mostcurrent physical addresses. The translation between logical to physicaladdresses would require, for example, a large centralized look-up FAT(file allocation table) to access the right slice in the right block onthe right chip. Such a search could add considerable read latency (e.g.,in the range of 50-100 microseconds) which would defeat a fast readaccess goal (e.g., under 100 nanoseconds). Accordingly, one aspect ofthe present invention significantly reduces the search time byintroducing a system-wide parallel on-chip rapid file searches, so as todramatically reduce the latency associated with a centralized large FAT,as described below.

Fast Reads: Pipelined Streaming and Random Access

At system initiation of a virgin multi-chip storage system of thepresent invention, all chips are erased and reference strings, referenceslices or reference planes are programmed to their reference states. Thesystem controller designates as cache storage the memory slices (e.g.,slice 116 in FIG. 6c ) that are physically closest to the senseamplifiers and voltage sources 206. Because of the RC delays along thelength of each NOR string, the TFTs in each string that are physicallyclosest to substrate circuitry 206 will have their voltages V_(bl)established a few nanoseconds sooner than the TFTs furthest fromsubstrate circuitry 206. For example, the first ˜50 slices or so (shownas slice 116 in FIG. 6c ) out of the 1K slices in each quadrant have theshortest latency and can be designated as a cache memory or storage, tobe used for storing quadrant operational parameters, as well asinformation regarding the files or data set stored in the quadrant. Forexample, each memory page (2×4 Kbits) or slice (2×4 Kbits×16=128 Kbits)written into the upper half-block (i.e., quadrants Q1 and Q2) can have aunique identifier number assigned to it by the system controller,together with an index number that identifies the type of file that isstored.

The cache storage may be used to store on-chip resource management data,such as file management data. A file can be identified, for example, as“hot file” (i.e., associated with a large number of accesses, or a “highcycle count”), “cold file” (i.e., has not been altered for a long time,and is ready to be moved to slower storage or archival memory at afuture time), “delete file” (i.e., ready for future erase in backgroundmode), “defective file” (i.e., to be skipped over), or “substitute file”(i.e., replacing a defective file). Also included in the identifier maybe a time stamp representing the last time and date the file associatedwith the identifier was written into the quadrant. Such uniqueidentifier, typically between 32-bit and 128-bit long can be writteninto one or more of the cache slices as part of the writing of the fileitself into the other memory slices in the same half-block. Files arewritten sequentially into available erased space, and the identifierscan be assigned by incrementing the previous unique identifier by onefor each new file written into memory. If desired, new files can bewritten into partial slices and the unwritten part of the slice can beused for writing part or whole of the next file, to avoid wastingstorage space. Writing sequentially until the entire memory space of thesystem is used helps level out the wear-out of TFTs throughout thesystem. Other on-chip resource management data may include chip, block,plane, slice, page and string parameters, address locations of faultystrings and their replacement strings, defective pages, defectiveplanes, defective slices and defective blocks and their substitutereplacements, file identifiers for all files resident in the block, lookup tables and link lists for skipping over unusable memory, block-erasecycle counts, optimum voltages and pulse shape and durations for erase,program, program-inhibit, program scrub, read, margin read, readrefresh, read scrub operations, error correcting codes, and datarecovery modes, and other system parameters.

Because of the modularity of each chip at the block level and the lowpower operation attendant to Fowler-Nordheim tunneling for program anderase, it is possible to design the chip to execute simultaneously eraseof some blocks, programming at some other blocks, and reading one ormore of remaining blocks. The system controller can use that parallelismof operations at the block level to work in background mode; forexample, the system controller may delete (i.e. erase, so as to free upspace) some blocks or entire chips, de-fragment fragmented files intoconsolidated files, move files, blocks or chips that have been inactivefor longer than a predetermined time to slower or archival storage, orto chips that group together files with close dates and time stamps,while rewriting the original file identifier with the latest time stampinto cache storage 116 of the next available physical block.

To facilitate high-speed searches for the location of the most currentversion of any one file out of the many millions such files in apetabyte storage system, it is important that the unique identifier foreach file, wherever it has been physically relocated to, be accessedquickly by the system controller. According to one embodiment of thepresent invention, a system controller broadcasts the unique identifier(i.e., the 32-128 bits word) for the file being searched simultaneouslyto some or all the chips in the system. Each chip is provided with abuffer memory to temporarily store that identifier and, using on-chipExclusive-Or (XOR) circuits, compare the identifier in the buffer memorywith all the identifiers stored on cache 116 of each block and report tothe system controller when a match has been found, together with thelocation where the corresponding file is located. If more than one matchis found, the system controller picks the identifier with the mostrecent time-stamp. The search can be narrowed to just a few chips if thefile being searched has been written within a known time period. For a1-terabit chip, just one 128-Kbit slice or 16×8 Kb pages would besufficient to store all the 64-bit identifiers for all 2K slices of eachblock.

TFT Pairs for Fast Read Cache Memory.

To reduce read latency for cache storage 116, TFTs in NOR strings thatare physically nearest to sense amplifiers 206 can be arranged in pairs.For example, in adjacent NOR strings, two TFTs related by a common localword line may be shared to store a single data bit between them. Forexample, in embodiment EMB-3 (FIG. 2k ), plane 202-7 includes a pair ofTFTs from adjacent active strips share local word lines 208-W (e.g., TFT281 on one NOR string can serve as a reference TFT for TFT 283, or viceversa). In a typical programming operation, TFTs on both NOR strings areinitialized to the erased state, then one of the TFTs, say TFT 281, isprogrammed to a higher threshold voltage, while TFT 283 isprogram-inhibited, so as to remain in the erased state. Both TFTs on thetwo adjacent active strips are read simultaneously by a differentialsense amplifier in substrate circuitry when their shared local word line208W is raised to the read voltage, the first TFT that start to conducttips the sense amplifier into state ‘0’ or state ‘1’, depending onwhether TFT 281 or TFT 283 is the programmed TFT.

This TFT-pair scheme has the advantage of high-speed sensing and higherendurance because TFTs of two adjacent NOR strings are almost perfectlymatched, so that at the sense amplifier even a small programmed voltagedifferential between the two TFTs being read will suffice to correctlytrip the sense amplifier. In addition, as the threshold voltage of aprogrammable reference TFT may drift over many write/erase cycle duringthe life of the device, under this scheme the reference TFT and the readTFT are both reset with each new cycle. In fact, either one of the twoTFTs in the pair can serve as the reference TFT. If the two TFTs makingthe pair are randomly scrambled to invert or not invert the data writtenin each cycle, to ensure that statistically each TFT in each pair servesas the reference TFT for approximately the same number of cycles as theother TFT. (The invert/not invert code can be stored in the same page asthe page being programmed, to assist in the descrambling during a readoperation). Because the paired TFTs are in close proximity to eachother, i.e., on two adjacent active strips on the same plane, the TFTscan best track each other for local variations in the manufacturingprocess or to best neutralize (i.e. cancel out) the strip leakage duringa read operation.

Alternatively, the TFT pairing scheme may be applied to TFTs ondifferent planes where the pair shares a common vertical local wordline. The one drawback of this scheme is that it cuts the siliconefficiency by nearly 50%, as the two TFTs are required to store one bitbetween them. For this reason, each block can be organized such thatonly a small percentage (e.g. 1% to 10%) of the block is used ashigh-speed dual TFT pairs, while the rest of the block is operated asregular NOR strings and programmable reference TFT strings. The actualpercentage set aside for the TFT-pair scheme can be altered on the flyby the system controller, depending on the specific usage application.The high level of flexibility for operating the NOR strings of thepresent invention result from the fact that the TFTs in a NOR string arerandomly addressable and operate independently of each other, or of TFTsin other NOR strings, unlike conventional NAND strings.

Numerous applications of data storage, such as video or high resolutionimaging require data files that occupy many pages or even many slices.Such files can be accessed rapidly in a pipelined fashion, i.e., thesystem controller stores the first page or first slice of the file inthe cache memory while storing the remaining pages or slices of the filein a low-cost memory and streaming out the data in a pipeline sequence.The pages or slices may thus be linked into a continuous stream, suchthat the first page of the file is read quickly into the senseamplifiers and transferred to a data buffer shift register to clock thefirst page out of the block while pre-charging and reading the next,slower page in a pipeline sequence, thereby hiding the read access timeof each page following the first page. For example, if the first page of8 Kbits stored in the cache memory is read in 10 nanoseconds and thenclocked out at 1 Gbit per second, the entire 8K bits would takeapproximately 1 microsecond to complete clocking out, which is more thansufficient time for the second page to be read from the slower,lower-cost pages. The flexibility afforded by pre-charging randomlyselected TFT strings makes it possible for one or more data files fromone or more blocks to be read concurrently, with their data streamsrouted on-chip to one or more data input/output ports.

Random Access Reads

The pre-charging scheme of the current invention allows data to beprogrammed to be serially clocked into, or randomly accessed, andlikewise read out serially in a stream or randomly accessed by words.For example, an addressed page in one plane can be read in one or moreoperations into the sense amplifiers, registers or latches of theaddressed plane, after which it can be randomly accessed in 32-bit,64-bit or 128-bit words, one word at a time, for routing to theinput/output pads of the chip. In this manner, the delay attendant tostreaming the entire page sequentially is avoided.

In all embodiments, for example FIG. 2h , only TFTs on one of the twosides of an active strip can participate in any one read operation;every TFT on the other side of an active strip must be set to the “off”state. For example, if TFT 285 is being read then TFT 283 on the sameactive strip must be shut off. Other schemes to read the correct stateof a multi-state TFT are known to those of ordinary skill in the art.

Reading TFTs of the present invention is much faster than readingconventional NAND flash memory cells because, in a NOR string, only theTFT to be read is required to be “on”, as compared to a NAND string, inwhich all the TFTs in series with the one TFT being read must also be“on”. In embodiments in which metallic sublayer 224 is not provided asintegral part of the active layer (see, e.g., memory structure 220 a ofFIG. 2b -1), for a string with 1,024 non-volatile TFTs on each side, atypical line resistance for each active strip is ˜500,000 Ohm and atypical capacitance of the active strip (e.g., capacitor 360 in FIG. 3a-1) is ˜5 femtofarads, to provide an RC time delay in the order of under10 nanosecond. The time delay may be significantly reduced if metallicsublayer 224 is provided to reduce the line resistance of the activestrip. To further reduce read latency, some or all the planes inselected memory blocks may be kept pre-charged to their read voltagesV_(ss) (source line) and V_(bl) (bit line), thereby rendering them readyto immediately sense the addressed TFT (i.e., eliminating the timerequired for pre-charge immediately before the read operation). Suchready-standby requires very little standby power because the currentrequired to periodically re-charge capacitor 360 to compensate forcharge leakage is very small. Within each block, all NOR strings on alleight or more planes can be pre-charged to be ready for fast read; forexample, after reading TFTs in NOR strings of plane 207-0 (FIG. 2a ),TFTs in NOR strings of plane 207-1 can be read in short order becauseits source and bit line voltages V_(ss) and V_(bl) are alreadypreviously set for a read operation.

In memory block 100, only one TFT per NOR string can be read in a singleoperation. In a plane with eight thousand side by side NOR strings, theeight thousand TFTs that share a common global word line may all be readconcurrently, provided that each NOR string is connected to its ownsense amplifier 206 in substrate 201 (FIG. 2c ). If each sense amplifieris shared among, for example, four NOR strings in the same plane using astring decode circuit, then four read operations are required to takeplace in four successive steps, with each read operation involving twothousand TFTs. Each plane can be provided its own set of dedicated senseamplifiers or, alternatively one set of sense amplifiers can be sharedamong NOR strings in the eight or more planes through a plane-decodingselector. Additionally, one or more sets of sense amplifiers can beshared between NOR strings in quadrants and their mirror image quadrants(see, e.g., sense amplifiers (SA) 206 in FIGS. 6a, 6b, and 6c ).Providing separate sense amplifiers for each plane allows concurrentread operations of NOR strings of all planes, which correspondinglyimproves the read operation throughput. However, such higher datathroughput comes at the expense of greater power dissipation and theextra chip area needed for the additional sense amplifiers (unless theycan be laid out in substrate 201 underneath block 100). In practice,just one set of sense amplifiers per stack of NOR strings may sufficebecause of the pipeline clocking or data in and out of the memory block,so that while a first page in one plane is being transferred out of itssense amplifiers to a high speed shift register, the first page of thesecond plane is being read into the second set of sense amplifiers, withthe two sets sharing one set of input/output shift registers.

Parallel operations may also create excessive electrical noise throughground voltage bounces when too many TFTs are read all at once. Thisground bounce is substantially suppressed in all embodiments that relyon pre-charging capacitor 360 to set and temporarily hold the virtualV_(ss) voltage for each active strip. In this case, source voltageV_(ss) of all NOR strings is not connected to the chip's V_(ss) groundline, allowing any number of active strips to be sensed simultaneouslywithout drawing charge from the chip ground supply

Program (Write) and Program-Inhibit Operations.

There are several methods to program an addressed TFT in a NOR string toits intended threshold voltage. The most common method, employed by theindustry for the past 40 years, is by channel hot-electron injection.The other commonly used method is by tunneling, whether direct tunnelingor Fowler-Nordheim tunneling. Either one of these tunneling andcharge-trapping mechanisms is highly efficient, so that very littlecurrent is needed to program a TFT in a NOR string, allowing parallelprogramming of hundreds of thousands of such TFTs with minimal powerdissipation. For illustration purpose, let us assume that programming bytunneling requires a 20V pulse of 100 microseconds (us) duration to beapplied to the addressed word line (control gate), with 0V applied tothe active strip (e.g., an active strip formed out of active layer 202-0in FIG. 2a ). Under these conditions, N⁺ sublayers 221 and 223 (FIG. 2b-1), serving respectively as source and drain regions, are both set at0V. P⁻ channel sublayer 222 of the TFT is inverted at the surface, sothat electrons tunnel into the corresponding charge-trapping layer. TFTProgramming can be inhibited by applying a half-select voltage (e.g.,10V in this example) between the local word line and the source anddrain regions. Program-inhibit can be accomplished, for example, eitherby lowering the word line voltage to 10V, while keeping the stripvoltage at 0 volt, or by raising to 10V the active strip voltage, whilekeeping the word line voltage at 20V, or some combination of the two.

Only one TFT in one addressed active strip can be programmed at onetime, but TFTs on other active strips can be programmed concurrentlyduring the same programming cycle. When programming one of the many TFTson one side edge of an addressed active strip (e.g., one TFT in theeven-addressed NOR string), all other TFTs in the NOR string areprogram-inhibited, as are all TFTs on the other side edge of the activestrip (e.g., all TFTs in the odd-addressed NOR string).

Once the addressed TFT is programmed to the target threshold voltage ofits designated state, program-inhibition of that TFT is required, asovershooting that target voltage will exert unnecessary stress on theTFT. When MLC is used, overshooting the target voltage may causeoverstepping or merging with the threshold voltage of the next highertarget threshold voltage state, and the TFT that has reached itsintended threshold voltage must therefore be program-inhibited. Itshould be noted that all TFTs in the adjacent active strips on the sameplane that share the same global word line and its associated local wordlines are exposed to the 20V programming voltage—and are required to beprogram-inhibited once they have been programmed to their targetthreshold voltages. Also, TFTs that are in the erased state and that areto remain erased need to be program-inhibited. Similarly, all TFTs onother planes that are within the same block and that share the sameglobal word line and its associated local word lines (i.e. all TFTs in aslice 114)—and thus, are also exposed to the 20V programming voltage—arealso required to be program-inhibited. These program and program-inhibitconditions can all be met for the memory blocks of the present inventionbecause the even and odd sides of each active strip are controlled bydifferent global word lines and their associated local word lines, andbecause the voltages on the shared source and bit lines of each activestrip regardless of its plane can be set independently from all otheractive strips on the same plane or on other planes.

In one example of a programming sequence, all TFTs in a block are firsterased to a threshold voltage of around 1V. The voltage on the activestrip of each addressed TFT is then set to 0V (e.g., through connections270 in conjunction with pre-charge word line 208-CHG, or throughhard-wire connections 280, as illustrated in FIG. 3a -1), if theaddressed TFT is to be programmed; otherwise, the voltage on the sharedsource line of the active strip of the addressed TFT is set to ˜10V ifit is to remain in its erased state (i.e., program-inhibited). Theglobal word line associated with the addressed TFT is then raised to˜20V, either in one step or in short-duration steps of incrementallyincreasing voltages, starting at around 14V. Such incremental voltagesteps reduce the electrical stress across the charge-trapping layer ofthe TFT and avoid overshooting the target programmed threshold voltage.All other global word lines in the block are set at half-select 10V. Allactive strips on all planes that are not being addressed in the memoryblock, as well as all active strips within the addressed plane that arenot individually addressed, are also set at 10V, where they may befloated by ensuring that their access transistors (not shown) tosubstrate circuitry 206-0 and 206-1 of FIG. 2c are off. Of importance,if any of the active strips on all planes that are not being addressedin the memory block, as well as all active strips within the addressedplane that are not individually addressed, are floated with theirvoltage set at ˜0V, i.e. not in program-inhibit mode, they may beerroneously programmed. These active strips are stronglycapacity-coupled to their local word lines, which are at 10V, and thusfloat at close to 10V. Each of the incrementally higher voltageprogramming pulses is followed by a read cycle to determine if theaddressed TFT has reached its target threshold voltage. When the targetthreshold voltage is reached, the active strip voltage is raised to ˜10V(alternatively the strip is floated, and rises close to 10V when all butthe one addressed global word lines in the block are raised to 10V) toinhibit further programming, while the global word line continues toprogram other addressed strips on the same plane that have not yetattained their target threshold voltages. This program/read-verifysequence terminates when all addressed TFTs have been read-verified tobe correctly programmed. All blocks on a chip that are dormant, i.e.they are not frequently accessed, should preferably be powered down, forexample by setting the voltage on their active strips and conductors atground potential.

When MLC is used, programming of the correct one of the multiplethreshold voltage states can be accelerated by parallel programming ofall target voltage states in parallel. First, capacitors 360 of alladdressed active strips (see, e.g., through connections 270 andpre-charge word lines 208-CHG of FIG. 3a -1) are pre-charged to one ofseveral voltages (e.g., 0, 1.5, 3.0, or 4.5V, if two bits of informationare to be stored in each TFT). A ˜20V pulse is then applied to theaddressed global word line, which expose the charge-trapping layers ofthe TFTs to different effective tunneling voltages (i.e., 20, 18.5, 17,or 15.5V, respectively), resulting in the correct one of the fourthreshold voltages being programmed in a single coarse programming step.Thereafter, fine programming pulses may be applied at the individual TFTlevel.

Because of the intrinsic parasitic capacitance C of every active stripin the block, all active strips on all planes in a block can have theirpre-charge voltage states set in place (either in parallel orsequentially) in advance of applying the high voltage pulsing on theaddressed global word line. Consequently, concurrent programming of agreat many TFTs can be achieved. For example, in FIG. 1a -2, all TFTs inone page 113, or all pages in one slice 114 can be course-programmed inone high voltage pulsing sequence. Thereafter, individual read-verify,and where necessary, resetting properly programmed active strips intoprogram-inhibit mode can be carried out. Pre-charging is advantageous,as programming time is relatively long (e.g., around 100 microsecond)while pre-charging all capacitors 360 or read-verifying of addressedTFTs can be carried out over a time period that is around 100nanoseconds, or 1,000 times faster. Thus, it is advantageous to programa large number of TFTs in a single global word line programmingsequence, and this is made possible because the programming mechanismsof direct tunneling or Fowler-Nordheim tunneling require only a smallcurrent per TFT being programmed. The programming typically requirestrapping a hundred or less electrons in the charge-trapping material toshift. The TFT threshold by one or more volts, and these electrons canreadily be supplied from the reservoir of electrons pre-charged onto theparasitic capacitor of the active string, provided that the string hassufficient number of TFTs contributing to parasitic capacitance.

It is important to note that, because of the poor efficiency ofprogramming TFTs with the conventional channel hot-electron injectionmechanism—requiring several orders of magnitude more electrons, ascompared to programming by tunneling—to adequately shift the thresholdvoltage of one TFT, channel hot-electron injection is not suitable foruse with embodiments relying on pre-charging multiple active strips.Instead, channel hot-electron injection programming requires hard-wiredconnections to the addressed source and drain regions duringprogramming, thus severely limiting the ability to perform parallelprogramming

Erase Operations

With some charge-trapping layers, erase is accomplished through eitherreverse-tunneling of the trapped electron charge or tunneling of holesto electrically neutralize the trapped electrons. Erase is slower thanprogramming and may require tens of milliseconds of erase pulsing.Therefore, the erase operation is frequently implemented at the block,or at the multiple blocks level, often in a background mode. The blocksto be erased are tagged to be pre-charged to their predetermined erasevoltages, followed by concurrently erasing all the tagged blocks anddiscontinuing erase of those blocks that have been verified to beproperly erased, while continuing to erase the other tagged blocks.Typically, block erase can be carried out by applying ˜20V to theP-sublayer 222 (FIG. 2b -1) of every active strip through connectionthrough pillars 290 (FIGS. 3a -1, 4 d, 2 k-1), while holding all globalword lines in the block at 0V. However, since pillars 290 cannot beemployed in embodiments where metallic sublayers 224 are used, as theyprovide a path for excessive leakage between different planes, onealternative method to erase all TFTs in the block in the absence ofsubstrate contact to P⁻ channels 222 is by doping the P⁻ sublayers 222to the relatively high range of 1×10¹⁷/cm³ to 1×10¹⁸/cm³ so as toincrease the N⁺P⁻ reverse bias conduction characteristics. Then, when N⁺sublayers 221 and 223 of all active strips that are to be erased areraised to ˜20V (through substrate connection 206-0 of FIG. 2c ), reversejunction leakage brings the voltage on P⁻ sublayers 222 (channel region)to close to 20V, initiating tunnel erase by ejecting electrons trappedin the charge-trapping layer into the P⁻ sublayer 222 for all TFTs withlocal word lines held at ˜0V.

Partial block erase is also possible. For example, if only TFTs on oneor more selected slices 114 (FIG. 6b ) are to be erased, pillars 290that typically are shared by all active strips in block 100 areconnected to the substrate circuitry (e.g., substrate circuitry 262-0 inFIG. 5b ) to supply the high erase voltage V_(erase) to the P⁻ sublayer222 (channels) of all TFTs in the block. The global word lines of allslices in the block other than the slices selected for erase are held athalf-erase voltage ˜10V or they are floated. The one or more slices tobe erased have their global word line brought to ˜0V for the duration ofthe erase pulse. This scheme requires that strip-select decoders employhigh voltage transistors that can withstand erase voltage V_(erase)˜20volts at their junctions. Alternatively, all but the addressed globalword line are held at zero volts, while pulsing the addressed globalword line to ˜20V supplied from the substrate and charging all activestrips in planes 202-0 through 202-7 to 0V. This method allowspartial-block erase of one or more Z-X slices 114 of all TFTs sharingthe addressed global word lines.

Other schemes are possible for partial block erase. For example, if oneor more selected Z-X slices is to be erased while all others are to beerase-inhibited; all global word lines in the block are first held at0V, while all strings in the block are charged from the substrate to thehalf-select voltage ˜10V and then are left isolated (floated) byswitching off their access select transistors (not shown) in substrate270. Then, all global word lines in the block are raised to ˜10V,thereby boosting the voltage on all active strings to ˜20V by capacitivecoupling. Then, the global word lines of the one or more Z-X slices tobe erased are brought to 0V while the remaining global word linescontinue to be held at 10V for the duration of the erase pulse. Notethat, to select active strips for partial block erase, their accesstransistors in substrate 270 may need to be high-voltage transistors,able to hold the ˜20V of charge on the active strip for a duration inexcess of the time required for the program or erase operation. Themagnitude and duration of erase pulses should be such that most TFTs areerased to a slight enhancement mode threshold voltage, between zero andone volts. Some TFTs may overshoot and be erased into depletion mode(i.e., having a slightly negative threshold voltage). Such TFTs arerequired to be soft-programmed into a slight enhancement mode thresholdvoltage subsequent to the termination of the erase pulses, as part ofthe erase sequence.

Fringing-Field Assisted Lateral Hopping Tunnel Erase in Highly ScaledShort-Channel TFTs.

As previously discuss in this disclosure, active strips of the presentinvention can be made with ultra-short channel TFTs (e.g., P⁻ sublayer522 of TFT T_(R) 585 of embodiment EMB-3A in FIG. 5g may have aneffective channel length L as short as 10 nm). FIG. 7 is a cross sectionin the Z-X plane of active layer 502-7 of embodiment EMB-3A, showing ingreater detail short-channel TFT T_(R) 585 of FIG. 5g , in which N⁺sublayer 521 serves as source and N⁺ sublayer 523 serves as drain and P⁻sublayer 522 serves as channel in conjunction with charge storagematerial 531 and word line 208W. FIG. 7 illustrates erasing TFTs of asufficiently short channel length L using the lateral-hopping of trappedelectrons mechanism within charge-trapping material 531-CT (as indicatedby arrow 577), accompanied by electron-tunneling into N⁺ sublayer 521and N⁺ sublayer 523 (as indicated by arrow 578) under the fringingelectric fields in ellipsoid space 574 that is provided by the voltage(˜0V) on word line 208W and the voltage (˜20V) on both N⁺ sublayers 521and 523.

As shown in FIG. 7, the charge-trapping layer 531 consists of tunneldielectric sublayer 531-T, Charge-trapping sublayer 531-CT (e.g.,silicon-rich silicon nitride), and blocking dielectric sublayer 531-B.Because of its very short channel length, the overlying channel (i.e.,P-sublayer 522) becomes strongly influenced by fringing electric fields(indicated in FIG. 7 by dashed ellipsoids 574) between local word line208W and N⁺ sublayer 521 (the source region) and N⁺ sublayer 523 (thedrain region).

During erase, electrons (indicated by dashed line 575) that are trappedin charge-trapping sublayer 531-CT are removed by tunneling, asindicated by arrows 573 and 576, to the source region (N⁺ sublayer 521)and the drain region (N⁺ sublayer 523), respectively, which are bothheld at a high erase voltage V_(erase) ˜20V. In some circumstances,voltage V_(erase) on P-channel 522 may be lower than ˜20V, particularlyif P⁻ pillars 290 are not provided, or are unable to supply the full˜20V from the substrate, so that tunnel-erase of electrons trapped closeto the P⁻ sublayer 522 may be less effective. However, fringing fields574 assist in lateral migration (i.e., sideways, as indicated by arrows577) of electrons in the silicon-rich silicon nitride of charge-trappingsublayer 531-CT. This lateral migration is often referred to as hoppingor Frankel-Poole conduction, resulting from electrons being attracted tothe ˜20V on the nearby source and drain regions. Once electrons havemigrated sufficiently close to the source and drain regions, theelectrons can tunnel out of charge-trapping sublayer 531-CT, asindicated by arrow 578. This fringing field-assisted erase mechanismbecomes increasingly more effective with shorter channel length (e.g.,in the range of 5 nanometers to 40 nanometers), provided thesource-drain leakage is tolerable for the short channel. Forhighly-scaled channel length, the source-drain leakage is suppressed bymaking the P⁻ sublayer 522 as thin as possible (e.g., in the range of 8to 80 nanometers thick), so that it is readily depleted all the waythrough its thickness, when the transistor is in its “off” state.

Quasi-Volatile Random Access TFT Memory Strings in Three DimensionalArrays.

The charge-trapping material (e.g., an ONO stack) described above has along data retention time (typically measured in many years), but lowendurance. Endurance is a measure of a storage transistor's performancedegradation after some number of write-erase cycles. Endurance of lessthan around 10,000 cycles is considered too low for some storageapplications requiring frequent data rewrites. However, the NOR stringsof embodiments EMB-1, EMB-2, and EMB-3 of the present invention may beprovided a charge-trapping material that substantially reduces retentiontimes, but significantly increases endurance (e.g., reducing retentiontime from many years to minutes or hours, while increasing endurancefrom ten thousand to tens of millions of write/erase cycles). Forexample, in an ONO film or a similar combination of charge-trappinglayers, the tunnel dielectric layer, typically 5-10 nm of silicon oxide,can be thinned to 3 nanometers or less, replaced altogether by anotherdielectric (e.g., silicon nitride or SiN) or no be simply eliminated.Similarly, the charge-trapping material layer may be a more silicon-richsilicon nitride (e.g., Si_(1.0)N_(1.1)), which is more silicon-rich thanconventional Si₃N₄. Under a modest positive control gate programmingvoltage, electrons may directly tunnel through the thinner tunneldielectric layer into the silicon nitride charge-trapping material layer(as distinct from Fowler-Nordheim tunneling, which typically requireshigher voltages to program). The electrons may be temporarily trapped inthe silicon nitride charge-trapping layer for a few minutes, a fewhours, or a few days. The charge-trapping silicon nitride layer and theblocking layer (e.g., silicon oxide, aluminum oxide, or other high-Kdielectrics) keep electrons from escaping to the control gate (i.e.,word line). However, the trapped electrons will eventually leak back outto N⁺ sublayers 221 and 223, and P⁻ sublayer 222 of the active strip, asthe electrons are negatively charged and repel each other. Even if the 3nm or less tunnel dielectric layer breaks down locally after extendedcycling, the trapped electrons are slow to depart from their traps inthe charge-trapping material.

Other combinations of charge storage materials may also result in a highendurance but lesser retention (“semi-volatile” or “quasi-volatile”)TFT. Such a TFT may require periodic write refresh or read refresh toreplenish the lost charge. Because the TFTs of embodiments EMB-1, EMB-2and EMB-3 provide DRAM-like fast read access time with low latency, byincluding any of the high endurance charge-trapping layers in the TFTs,NOR string arrays having such TFTs may be used in some applications thatcurrently require DRAMs. The advantages of such NOR string arrays overDRAM include: a much lower cost-per-bit because DRAMs cannot be readilybuilt in three-dimensional blocks, and a much lower power dissipation,as the refresh cycles need only be run approximately once every fewminutes or once every few hour, as compared to every ˜64 millisecondsrequired in current DRAM technology. Quasi-volatile embodiments of theNOR string arrays of the present invention appropriately adapt theprogram/read/erase conditions to incorporate the periodic datarefreshes. For example, because each quasi non-volatile TFT isfrequently read-refreshed or program-refreshed, it is not necessary to“hard-program” TFTs to provide a large threshold voltage window betweenthe ‘0’ and ‘1’ states that is typical for non-volatile TFTs where aminimum 10 years data retention is required. For example, aquasi-volatile threshold voltage window can be as little as 0.2V to 1V,as compared to 1V to 3V typical for TFTs that support 10-yearsretention.

Read, Program, Margin Read, Refresh and Erase Operations forQuasi-Volatile Nor Strings.

The quasi-volatile NOR strings or slices of the current invention may beused as alternatives to some or all DRAMs in many memory applications,e.g., the memory devices for supporting central processing unit (CPU) ormicroprocessor operations on the main board (“motherboard”) of acomputer. The memory devices in those applications are typicallyrequired to be capable of fast random read access and to have very highcycle-endurance. In that capacity, the quasi-volatile NOR strings of thepresent invention employ similar read/program/inhibit/erase sequences asthe non-volatile NOR implementation. In addition, since the chargestored on programmed TFTs slowly leaks out, the lost charge needs to bereplenished by reprogramming the TFTs in advance of a read error. Toavoid the read error, one may employ “margin read” conditions todetermine if a program-refresh operation is required, as are well knownto a person skilled in the art. Margin read is an early-detectionmechanism for identifying which TFT will soon fail, before it is toolate to restore it to its correct programmed state. Quasi-volatile TFTstypically are programmed, program-inhibited or erased at reducedprogramming voltage (V_(pgm)), program inhibit voltage (V_(inhibit)) orerase voltage (V_(erase)), or are programmed using shorter pulsedurations. The reduced voltages or shorter pulse durations result in areduced dielectric stress on the storage material and, hence,improvement by orders of magnitude in endurance. All slices in a blockmay require periodic reads under margin conditions to early-detectexcessive threshold voltage shifts of the programmed TFTs due to chargeleakage from their charge storage material. For example, the erasethreshold voltage may be 0.5V±0.2 V and the programmed threshold voltagemay be 1.5V±0.2V, so that a normal read voltage may be set at ˜1V whilethe margin-read may be set at ˜1.2V. Any slice that requires aprogram-refresh needs to be read and then correctly reprogrammed intothe same slice or into an erased slice in the same block or in anotherpreviously erased block. Multiple reads of quasi-volatile TFTs canresult in disturbing the erase or program threshold voltages, and mayrequire rewriting the slice into another, erased slice. Read disturbsare suppressed by lowering the voltages applied to the control gate, andthe source and drain regions during reads. However, repetitive reads maycumulatively cause read errors. Such errors can be recovered byrequiring the data to be encoded with error correcting codes (“ECC”).

One challenging requirement for the proper operation of thequasi-volatile memory of the present invention is the ability to readand program-refresh a large number of TFTs, NOR strings, pages orslices. For example, a quasi-volatile 1-terabit chip has ˜8,000,000slices of 128K bits each. Assuming that 8 slices (˜1 million) of TFTscan be program-refreshed in parallel (e.g., one slice in each of 8blocks), and assuming a program-refresh time of 100 microseconds, thenan entire chip can be program-refreshed in ˜100 seconds. This massiveparallelism is made possible in memory devices of the present inventionprimarily because of two key factors; 1) Fowler-Nordheim tunneling ordirect tunneling requires extremely low programming current per TFT,allowing an unprecedented 1 million or more TFTs to be programmedtogether without expanding excessive power; and 2) the parasiticcapacitor intrinsic to a long NOR string enables pre-charging andtemporarily holding the pre-charged voltage on multiple NOR strings.These characteristics allow a multitude of pages or slices on differentblocks to be first read in margin-read mode to determine if a refresh isrequired, and if so, the pages or slices are individually pre-chargedfor program or program-inhibit and then program-refreshed in a singleparallel operation. A quasi-volatile memory with average retention timeof ˜10 minutes or longer will allow the system controller to haveadequate time for properly program-refresh, and to maintain a low errorrate that is well within the ECC recovery capability. If the entire1-terabit chip is refreshed every 10 minutes, such a chip comparesfavorably with a typical 64 milliseconds-to-refresh DRAM chip, or afactor of more than 1,000 times less frequently, hence consuming farless power to operate.

FIG. 8a shows in simplified form prior art storage system 800 in whichmicroprocessor (CPU) 801 communicates with system controller 803 in aflash solid state drive (SSD) that employs NAND flash chips 804. The SSDemulates a hard disk drive and NAND flash chips 804 do not communicatedirectly with CPU 801 and have relatively long read latency. FIG. 8bshows in simplified form system architecture 850 using the memorydevices of the current invention, in which non-volatile NOR stringarrays 854, or quasi-volatile NOR string arrays 855 (or both) areaccessed directly by CPU 801 through one or more of input and output(I/O) ports 861. I/O ports 861 may be one or more high speed serialports for data streaming in or out of NOR string arrays 854 and 855, orthey may be 8-bit, 16-bit, 32-bit, 64-bit, 128-bit, or any suitablysized wide words that are randomly accessed, one word at a time. Suchaccess may be provided, for example, using DRAM-compatible DDR4, andfuture higher speed industry standard memory interface protocols, orother protocols for DRAM, SRAM or NOR flash memories. I/O ports 862handle storage system management commands, with flash memory controller853 translating CPU commands for memory chip management operations andfor data input to be programmed into the memory chips. In addition, CPU801 may use I/O ports 862 to write and read stored files using one ofseveral standard formats (e.g., PCIe, NVMe, eMMC, SD, USB, SAS, ormulti-Gbit high data-rate ports). I/O ports 862 communicate betweensystem controller 853 and NOR string arrays in the memory chips.

It is advantageous to keep the system controller (e.g., systemcontroller 853 of FIG. 8b ) off the memory chips, as each systemcontroller typically manages a number of memory chips, so that it isdisengaged as much as possible from the continuous ongoingmargin-read/program-refresh operations, which can be more efficientlycontrolled by simple on-chip state machines, sequencers or dedicatedmicrocontrollers. For example, parity-check bit (1-bit) or more powerfulECC words (typically, between a few bits to 70 bits or more) can begenerated for the incoming data by the off-chip controller or on-chip bydedicated logic or state machines and stored with the page or slicebeing programmed During a margin-read operation the parity bit generatedon-chip for the addressed page is compared with the stored parity bit.If the two bits do not match, the controller reads again the addressedpage under a standard read (i.e. non-margin). If that gives a parity bitmatch, the controller will reprogram the correct data into the page,even though it is not yet fully corrupted. If the parity bits do notmatch, then on-chip dedicated ECC logic or the off-chip controller willintervene to detect and correct the bad bits and rewrite the correctdata preferably into another available page or slice, and permanentlyretiring the errant page or slice. To speed up the on-chip ECCoperations, it is advantageous to have on-chip Exclusive-Or, or otherlogic circuitry to find ECC matches quickly without having to gooff-chip. Alternatively, a memory chip can have one or more high-speedI/O ports dedicated for communication with the controller for ECC andother system management chores (e.g., dynamic defect management), so asnot to interfere with the low latency data I/O ports. As the frequencyof read or program-refresh operations may vary over the life of thememory chip due to TFT wear-out after excessive program/erase cycling,the controller may store in each block (preferably in the high-speedcache slices) a value indicating the time interval between refreshoperations, This time interval tracks the cycle count of the block.Additionally, the chip or the system may have a temperature monitoringcircuit whose output data is used to modulate the frequency of refresheswith chip temperature. It should be clear that the example used here isjust one of several sequences possible for achieving automaticprogram-refresh with rapid correction or replacement of errant pages orslices.

In the example of a 1-terabit chip having only 8 blocks out of 4,000blocks, or 0.2% or less of all blocks are being refreshed at any onetime, program-refresh operations can be performed in a background mode,while all other blocks can proceed in parallel with their pre-charge,read, program and erase operations. In the event of an address collisionbetween the 0.2% and the 99.8% of blocks, the system controllerarbitrates one of the accesses is more urgent. For example, the systemcontroller can interrupt a program-refresh to yield priority to a fastread, then return to complete the program-refresh.

In summary, in the integrated circuit memory chip of the presentinvention, each active strip and its multiple associated conductive wordlines are architected as a single-port isolated capacitor that can becharged to pre-determined voltages which are held semi-floating (i.e.,subject to charge leaking out through the string-select transistor inthe substrate circuitry) during read, program, program-inhibit or eraseoperations. That isolated semi-floating capacitor of each active strip,coupled with the extremely low Fowler-Nordheim or direct tunnelingcurrent required to program or erase the TFTs in a NOR string associatedwith the active strip, makes it possible to program, erase or read amassive number of randomly selected blocks, sequentially orconcurrently. Within the integrated circuit memory chip, the NOR stringsof one or more of a first group of blocks are first pre-charged and thenerased together, while the NOR strings of one or more other groups ofblocks are first pre-charged and then programmed or read together.Furthermore, erasing of the first group of blocks and programming orreading of a second group of blocks can take place sequentially orconcurrently. Blocks that are dormant (e.g., blocks that storerarely-changed archival data) are preferably held at a semi-floatingstate, preferably isolated from the substrate circuits after havingtheir NOR strings and conductors set at ground potential. To takeadvantage of the massively parallel read and program bandwidths of thesequasi-floating NOR strings, it is advantageous for the integratedcircuit memory chip to incorporate therein multiple high-speed I/Oports. Data can be routed on-chip to and from these I/O ports, forexample, to provide multiple channels for word-wide random access, orfor serial data streams out of the chip (reading) or into the chip(programming or writing).

Fast Logic Operations and Analog Operations

Many applications (e.g., search, machine learning and numerous otherartificial intelligence applications) require fast Boolean operationsinvolving numerous binary variables. For example, a search applicationoften requires matching of keys to ascertain that the result found isthe data item sought. The NOR memory strings of the present inventionmay be used to implement fast Boolean operations involving a largenumber of Boolean variables. For example, the NOR memory stringsdescribed above may be used to compare many bits in parallel. Such acompare function may be implemented using two n-bit NOR memory strings.Consider a Boolean string a_(n-1) a_(n-2) . . . a₀, which is to becompared to an input Boolean string b_(n-1) b_(n-2) . . . b₀. a_(n-1)a_(n-2) . . . a₀. Such a comparison is often required, for example, inapplications involving look-up tables, content addressable memories,cache tag hit/miss detections, or key searches associated with datastored in hashed locations. In one embodiment, the Boolean stringa_(n-1) a_(n-2) . . . a₀ may be stored in two NOR memory string,referred to as “true-string” and “complement-string,” respectively, inthe following manner: (i) a ‘1’ in Boolean string a_(n-1) a_(n-2) . . .a₀ is stored in a non-conducting state (e.g., high threshold voltagestate or N) in the true-string and a ‘0’ in Boolean string a_(n-1)a_(n-2) . . . a₀ is stored in the true-string as a conducting state(e.g., a low threshold voltage state or C), and (ii) a ‘0’ in Booleanstring a_(n-1) a_(n-2) . . . a₀ is stored in the non-conducting state inthe complementary-string and a ‘1’ in Boolean string a_(n-1) a_(n-2) . .. a₀ is stored in the complementary-string as a conducting state. Table1 illustrates this programming scheme for Boolean string ‘11 . . . 011’:

string a_(n−1) a_(n−2) . . . a₂ a₁ a₀ True-string N N . . . C N NComplementary- C C . . . N C C string

When a bit is read at the true-string, a stored ‘1’ (non-conductingstate) would result in a high voltage on the common bit line of the NORmemory string, as no current flow is seen across the common bit line andthe common source terminal, whereas a stored ‘0’ (conducting state)would result in a low voltage, as the current flow pulls the voltage onthe common bit line to the voltage at the common source terminal.Conversely, when a bit is read in the complementary-string, a stored ‘1’(conducting state) would result in a low voltage on the common bit line,whereas a stored ‘0’ (non-conducting state) would result in a highvoltage on the common bit line.

To perform the compare operation, a ‘1’ bit in input Boolean stringb_(n-1) b_(n-2) . . . b₀ results in reading the corresponding bit in thetrue-string, and a ‘0’ in input Boolean string b_(n-1) b_(n-2) . . . b₀results in reading the corresponding bit in the complementary-string.The read operations of every bit in Boolean string b_(n-1) b_(n-2) . . .b₀ are all performed simultaneously by simultaneously activating thecorresponding word lines. Hence, if stored Boolean string a_(n-1)a_(n-2) . . . a₀ and input Boolean string b_(n-1) b_(n-2) . . . b₀ arenot identical, at least one of the bits read in either the true-stringor the complementary-string would be in the conducting state, resultingin a low voltage on the common bit line of that NOR memory string.However, if stored Boolean string a_(n-1) a_(n-2) . . . a₀ and inputBoolean string b_(n-1) b_(n-2) . . . b₀ are identical, neither of thecommon bit lines of the true-string and the complementary-string wouldbe in the conducting state, resulting in a high voltage in the commonbit line terminals of the true-string and the complementary-string. Thecompare operation described above performs the Boolean function:Π_(i=0) ^(n-1)(a _(i) b _(i) +ā _(i) b _(i))

As a NOR memory string of the present invention may have hundreds oreven thousands of memory cells, a large number of Boolean variable-paircomparisons may be performed in a single read cycle. Other Booleanfunctions involving large numbers of Boolean variables may beconstructed in like manner using the NOR memory strings of the presentinvention. For example, if one is interested only in matching ‘1’s inthe Boolean strings, the complementary-string can be omitted from theabove implementation. These logic functions provide significantadvantages when used in conjunction with the fast reads, pipelinedstreaming and random-access memory operations described above.

Other applications, e.g., certain artificial intelligence applications,may require generation of one of analog signals. In some applications,multiplications and additions may be performed very rapidly in theanalog domain when high precision is not required. The NOR memorystrings of the present invention may be used to generate analog signals.As shown in FIG. 3a -2, a conducting current in any of memorytransistors 222 in the NOR memory string of active layer 202-3, forexample, is determined by (i) voltage drop V_(bl)-V_(SS) between commonbit line 223 at terminal 270 and common source line 221 at terminal 280and (ii) the resistance along the current path. (Terminals 270 and 280may each be a reference voltage source or the ground reference, all ofwhich may be provided in semiconductor substrate 201). As discussedabove, common bit line 223 of FIG. 3a -2 is rendered low resistance byits contact with adjacent metallic layer 224. Without being in contactwith a similar metallic layer, the resistance along common source line221 increases with the distance the conducting memory transistor isfurther away from terminal 280. Voltage drop V_(bl)-V_(SS) is seensubstantially completely along source line 221 due to its higherresistance relative to common bit line 223. Labeling the memorytransistors in the n-bit NOR memory string from 0 to n−1, the conductingcurrent i_(k) in the k-th memory transistor may be expressed as:

${i_{k} = {\frac{V_{bl} - V_{SS}}{\left( {k + 1} \right)R} = \frac{K}{k + 1}}},$where

$K = {\frac{V_{bl} - V_{SS}}{R}.}$Suppose the programmed states of the memory cells in the n-bit NORmemory string is represented by binary string b_(n-1) b_(n-2) . . . b₀,such that the k-th memory transistor is programmed in the conductingstate, if b_(k) is ‘1’ and is programmed in the non-conducting state, ifb_(k) is ‘0’. Then, the total current I in the n-bit NOR memory string,when all the bits are read simultaneously, would be given by:

$I = {\sum\limits_{k = 0}^{n - 1}\;\frac{b_{k}K}{k + 1}}$

Thus, a current representing a desired analog value can be generated byprogramming the memory cells of a NOR memory string selectively inconducting and non-conducting states. The generated analog signal mayparticipate in computation in the analog domain using appropriate analogcircuitry provided under the array of NOR memory strings or in aseparate, accompanying integrated circuit. Conversion between theBoolean string and its corresponding analog value may be convenientlyaccomplished using, for example, look-up tables. One may recognize thatthe total current I may represent a weighted sum, with each weight

$0.0 < \frac{1}{k + 1} < 1.0$appropriate for representing a probability. Such a weighted sum is oftencomputed in the neurons of a neural network, which is widely used inmany machine learning and other artificial intelligence applications.Thus, the NOR memory strings of the present invention are particularlypowerful when used in many such applications.

In another embodiment, the resistance in the common bit line is notdiminished by an adjacent metallic layer. In that case, the current ineach conducting state memory transistor is substantially the same. Inthat embodiment, e.g., the NOR memory strings shown in FIG. 3a -1, thetotal current I is given by:

$I = {\sum\limits_{k = 0}^{n - 1}\;{b_{k}K}}$

Thus, such a NOR memory string is suitable for rapidly and efficientlygenerating an analog signal whose magnitude varies linearly with thenumber of memory cells programmed in the conducting state.

A distinct advantage of the current invention comes from the fact thatthe NOR memory strings of the current embodiments can be builtefficiently in three-dimensional memory stacks of such NOR memorystrings. In such configuration, the cost of each such string isdrastically reduced. For example, in one embodiment implemented on asingle semiconductor die, each three-dimensional memory stacks mayinclude, for example, eight or more active layers that can form NORmemory strings. Such a die may be organized into 1024 (1K) compactmodular units or “tiles,” with each tile having 16,385 (16K)non-volatile or quasi-volatile NOR memory strings of the types describedabove, for a total of more than 16 million such NOR memory strings, eachrepresenting an individual signal level. The tiles are each preferablyof a regular shape to facilitate layout and signal routing. In someapplications it may be advantageous to have the thin-film transistors ofeach NOR memory string be of the non-volatile type, specifically tostore data that only change infrequently. In other applications it maybe advantageous to have the stored data changes very frequently. Inthose case, as the thin-film transistors is required to have very higherase/write endurance, the quasi-volatile type transistors are bettersuited. (As discussed above, quasi-volatile thin-film transistors mayneed to be periodically read-refreshed.) In yet another embodiment ofthe present invention, the thin-film transistors in the NOR memorystrings of some of the tiles may be configured to be of the non-volatiletype, while the thin-film transistors in the NOR memory strings of othertiles may be configured to be of the quasi-volatile type.

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modification within the scope of the presentinvention are possible. The present invention is set forth in theaccompanying claims.

We claim:
 1. A method for generating an analog signal representative ofan input Boolean string, comprising: providing a plurality of memorycells wherein (i) the memory cells share a common drain region and acommon source region in a NOR memory string configuration, wherein oneor both of the common source region and the common drain region beingprovided as a substantially uniform strip of conductive semiconductormaterial, and (ii) the memory cells are separated from each othersubstantially uniformly along the substantially uniform strip ofconductive semiconductor material; associating each bit of the Booleanstring to a selected one of the memory cells; programming each selectedmemory cell to a conductive state or a non-conductive state according tothe value of the associated bit in the Boolean string; and reading theselected memory cells simultaneously to obtain a current flowing in thesubstantially uniform strip of semiconductor material and providing thatcurrent as the analog signal, wherein either the common drain region orthe common source region, but not both, is provided a lower resistancerelative to the other, such that the resulting current for any of theselected memory cells is also determined in part by where along thesubstantially uniform strip of semiconductor material is that memorycell located.
 2. The method of claim 1, wherein the lower resistance isprovided by a metallic layer provided adjacent to the common sourceregion or the common drain region.
 3. The method of claim 1, wherein theanalog signal is representative of a weighted sum, with each weightdetermined by where a corresponding one of the selected memory cells islocated along the substantially uniform strip of semiconductor material.4. The method of claim 1, wherein the selected memory cells implement aneuron in a neural network.
 5. The method of claim 1, wherein the memorycells are organized into a plurality of 3-dimensional arrays of memorycells provided above a semiconductor substrate.
 6. The method of claim5, wherein the 3-dimensional arrays are organized into tiles.
 7. Themethod of claim 6, wherein the memory cells in a selected group of tilesare configured to be non-volatile.
 8. The method of claim 6, wherein thememory cells in a selected group of tiles are configured to bequasi-volatile.
 9. The method of claim 1, wherein the analog signal isprovided as an input signal to analog circuitry.
 10. The method of claim1, wherein the analog circuitry is provided in the semiconductorsubstrate or in an accompanying integrated circuit.